New fire alarms for old buildings

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This is an existing system that was going to be upgraded. This whole panel is filled with relays to get certain functions accomplished such as releasing security doors, roll-down shutters, and smoke control systems. This type of panel is replaced by smaller devices or can be integrated with the building management system (BMS) such that the fire alarm only provides an output to the BMS and the BMS triggers all these functions. All photos: Arup

The prospect of retrofitting an existing fire alarm system can be very troubling to building owners and facility managers because of the many possible pitfalls. Apart from potential interruptions to normal business operations, upgrading a fire alarm system can cause a great deal of anxiety regarding design and installation costs.

Because building owners and operators are understandably concerned about the value that they are getting when upgrading their fire alarm systems, they naturally opt for curtailing planning to get an expedient installation. However, well-executed due-diligence planning not only has the potential to save substantial sums of money on first costs, but can result in significant long-term savings associated with reduced upkeep and maintenance.

To upgrade—or not

There are many reasons why a building owner or facility manager may want to upgrade or even replace an existing fire alarm system. A few of the more common reasons are discussed below.

Code compliance—The need to comply with federal, state, and municipal codes or requirements is perhaps one of the most common motivations for replacing or upgrading fire alarm systems. Although buildings that undergo a change in use or occupancy would certainly be expected to comply with the current requirements for the design and installation of fire alarm systems, even fairly minor renovations to a building can trigger fire alarm upgrades. Specifically, compliance with state and federal accessibility requirements may necessitate that notification appliances be brought up to current code even in areas that are not directly being altered.

Furthermore, accessibility upgrades are not only triggered as a result of voluntary alterations or additions, but also can be required during the course of repairs to a building following a fire, earthquake, or similar catastrophic event.

Wear and tear—Older high- and low-voltage systems that have been in operation for many years may start to show their age through an increasing number of system troubles, false alarms, or system down-time events. These occurrences reveal more fundamental underlying reliability concerns associated with aging or poorly maintained system components. Some of these components, such as power supplies, tend to wear out quicker than other installed elements of hardwired systems.

Other components, such as smoke detectors, may start to malfunction because they become dirty or damaged from the normal wear and tear that occurs in a lived-in building. Although many manufacturers recommend replacement of detection devices every 10 years, it is not uncommon for smoke detectors to be in service for more than twice as long. Over the span of many years, tenant improvements and other building alterations or system adjustments may have been performed, resulting in subpar performance.

Obsolescence—System obsolescence is another factor that may influence the decision to replace an existing fire alarm system. With older systems that did not rely as much on integrated circuit technology, a fire alarm system would last for 30 to 40 years. However, as the capabilities of fire alarm systems increased over time (and the reliance on proprietary technologies), the useful life span of the systems diminished. In much the same way that consumers have opted for newer, faster computers in the past two decades, so has the fire alarm industry become more reliant on rapidly advancing technologies that are not always backward-compatible or cross-listed for use with available equipment. The net result was that fire alarm systems that used to last three or four decades were now becoming obsolete in 10 to 20 years as manufacturers stopped making parts, opting to create ever more versatile systems. The lack of parts availability and associated compatibility challenges have led many owners to simply upgrade rather than deal with a perpetually outdated system.

Integration with other systems—A newer, more versatile fire alarm system may be required to more efficiently integrate fire alarm functions with other building systems such as security, access control, or BAS. Newer codes allow for greater integration of the fire alarm system with other building systems. The form of integration may simply consist of monitoring other building equipment and controls, or it may entail the development of a common communications infrastructure to serve multiple building systems.

The cost of keeping an old system

Apart from the increased flexibility associated with newer components, one of the most compelling reasons to retrofit an older fire alarm system is the cost of keeping the outdated system intact. Aging and obsolete systems are subject to a number of pitfalls that can manifest as a slow but steady trickle from the owner's coffers.

False alarms—Older systems may not be able to take advantage of new technologies such as self-adjusting intelligent detectors that use alarm verification functions or integrate several detection technologies into one device to read “fire signatures.” As a result, the occurrence of false alarms may be greater, potentially resulting in fire department fines or increased fees from third-party alarm monitoring/service companies.

Maintenance costs—Existing infrastructure may require more frequent cleaning of sensing devices, as well as repair of wiring and notification appliances due to vandalism, age, or poor-quality changes made to the system. The normal wear and tear associated with even the small changes related to tenant fit-outs can add up to large service bills. Additionally, because older fire alarm systems cannot readily be combined with other building systems, disparate infrastructure and multiple vendors become necessary, and with them, multiple service contracts.

System downtime—For obsolete systems in particular, there can be significant delays in getting replacement parts. If the system becomes partially or totally nonoperational, the authorities having jurisdiction may require a fire watch or otherwise limit building occupancy until the fire alarm system is functional. Clearly, such outcomes can be very costly to any business.

Improper installation of a smoke detector is shown. Also note: Over time, the detector has been painted and no longer mounted in the proper orientation. Use of wire and nail as conduit strap is not an acceptable mounting technique.   

 Planning for a new system

The first step in evaluating an existing fire alarm system is to pull together a team that knows code requirements and design approaches, inclusive of how fire alarm systems are installed. Typically, such a team consists of the owner or owner's representative, an engineer or fire alarm system designer with good working knowledge of fire alarm systems and the associated scope requirements, and a general contractor that can simultaneously direct the efforts of fire alarm contractors and service personnel.

Next, the team should undertake a due-diligence exercise to identify the relevant code requirements. It is critical to research basic code requirements to understand precisely how they pertain to the building occupancy and type. Although this might seem like an obvious task, it is often overlooked in favor of making assumptions about the existing uses that may or may not be true. Jurisdictions often allow a fire alarm system to be maintained in the arrangement in which it was originally approved, but many have different trip points as to when fire alarm maintenance becomes an upgrade that would require the entire existing system to be brought up to current code. A small investment in time to research the building occupancy and corresponding requirements early on can save huge sums of money and eliminate nasty surprises later during the plan review process.

After the basic code parameters have been identified, the team should do a bit of research about the system by examining drawings, interviewing maintenance personnel, and even reviewing available specifications in some instances. Such research might involve these types of questions:

• Was the system well-maintained and was maintenance documented, or are there potential unknowns that could impact cost, schedule, and operational integrity?
• Where the old and new systems are proposed to interface, will there be issues associated with strobe synchronization or the joining of other system components? Will information need to be shared between the two systems? It should be noted that most codes do not allow two systems to be permanently installed in a single building.
• Where the fire alarm system is integrated with other building systems, is the extent of the integration understood? Or are there potential gaps in knowledge or scope that can result in costly additions later? Systems that may be interconnected to the fire alarm system include building automation, smoke control, sprinkler (e.g., pre-action releasing valves), gaseous suppression, security, door unlocking or releasing, and elevator recall.
• Will the system likely be upgraded to include mass notification capabilities in the future?
• Are there requirements associated with the new system that could cause complications for the existing system? For example, if a voice alarm system was not required at the time of original system installation, but is now required, how does this impact the existing panels and speakers?
• Were there changes in related systems (e.g., fire sprinklers) that could cause complications? Would the changes in the new fire alarm system impact these systems, or vice-versa?
• Are accessibility requirements along the path to the modified areas from the building entrance also being addressed? What changes do potential accessibility upgrades necessitate?
• Are there any special occupancy requirements that would impact the contractor's ability to perform the work? For example, if the facility has to remain open during replacement, more costly measures should isolate building occupants from the construction.

The survey

Once the potential design (and cost) impacts have been identified, and the basic code research has been completed, it's time for the survey. The purpose of the survey is as much to identify existing conditions that will have an impact on the extent and constructability of any new fire alarm infrastructure, as to learn how much of the existing equipment can be reused. The survey can be framed in terms of the key system elements, including the backbone, initiating devices, and notification appliances. At a minimum, the following elements should be examined:

Fire alarm control panel—The manufacturer and model number of the panel are of equal import to the availability of expansion slots (the means by which additional appliances or devices are added). With this information in hand, it is possible to determine not only the technical capabilities of the system (e.g., is voice alarm an option with the existing panel?), but also the practical capabilities with respect to expansion of the fire alarm system.

Consideration must also be given to the existing system's manufacturer. Proprietary systems often require that the same manufacturer be used if parts of the system are to be reused. Also, if the system is part of a larger campus or site-wide system, the proprietary nature of the components will need to be addressed to maintain full functionality of the campus-wide system.

Wiring—Noting how the wiring was run, and whether conduit was provided, will go a long way toward informing the cost of adding new devices or appliances. Conduits can make it easier to run new wires through what would otherwise be very challenging post-construction environments. It should also be noted that while it may be attractive to retain existing wiring, it is not always the wisest decision because there may be compatibility issues between the wiring and any new installed equipment. If the existing system has many trouble conditions, they are often due to the wiring. All wire proposed to be reused and determined to be compliant with manufacturer's requirements should be tested for continuity and proper grounding before reuse.

Notification appliances—Speakers and strobes should be examined for quantity (coverage), intensity, and location. Newer codes typically have more stringent requirements for the spacing of notification appliances. Strobes manufactured prior to the Americans with Disabilities Act of 1990 (ADA) may have to be replaced with newer xenon strobes having greater flash intensity. Furthermore, systems installed before the enactment of the ADA may not be located at the proper heights, thus necessitating potentially expensive refinishing work to relocate speaker/strobe appliances. Strobe synchronization may also be of concern, both in terms of the existing system appliances and in terms of interfaces between subzones controlled by distinct panels. If a new front-end panel is being installed, cross-listing of existing appliances will need to be confirmed prior to re-use.

Power supplies—Power supplies should be looked at in concert with the notification appliances. Frequently, upgrades to the existing speaker/strobe system will necessitate upgrades to the power supplies serving a particular fire alarm system.

Detection—Because detectors must be replaced more frequently than other fire alarm system components, their condition and age should be carefully examined. To the extent possible, the surveyor should attempt to understand the history of the detectors to identify if trouble alarms or false alarms have resulted from age or improper application. Newer analog devices often use proprietary communications such that they cannot be re-used with other systems.

Alternatively, where older style four-wire detectors are still in use, the surveyor should be aware of potential cross-listing issues associated with the reuse of old style analog detectors with newer replacement panels. Although technically feasible to couple analog detection devices to newer fire alarm panels, such panels may not have been listed for use with the existing detectors, and thus the integrity of the system could be jeopardized.

Architectural features—The surveyor should take note of any aesthetic or historical architectural features that might require kid gloves. Common examples of features that may seem harmless at first but can result in significant cost impacts later are plaster or stone walls that have the potential to make running new wire very difficult. Similarly, relocating pull stations to the proper ADA height may require extensive stone work or less aesthetically pleasing cover plates. Alternative detection technologies such as air sampling and beam detection should be considered as they may allow the detection devices to be more easily concealed or limited in number, respectively.

Hazardous conditions—The presence of asbestos in the vicinity of fire alarm system components can pose significant cost and logistical challenges. If it becomes necessary to interact with existing asbestos installations, additional specialists will likely have to be hired to safely mitigate potential asbestos exposure.

The meeting of the minds

Because each jurisdiction has different requirements for the extent of work required to integrate system components or to update a fire alarm system, the team should have a meeting to flesh out the details.

For example, the design team will generally have to obtain concurrence that not all existing systems being touched will have to be brought up to code simply because portions of those systems are being upgraded in one way or another. Spelling out exactly what is being done and what is not being done can help to avoid nasty surprises during the inspection process that could impact the successful completion of the project.

Bringing it all together

The final step of the process is developing a fire alarm narrative and associated bid documents. Depending on the scope, scale, and location, these documents may be detailed drawings and specifications, or schematic sketches and performance specifications designed to facilitate the development of competitive bids for the fire alarm replacement or upgrade. In addition to their role in reducing risk to the bidders, excellent narratives, drawings, and specifications can be used to better articulate complex aspects of the desired system design, such as fire alarm integration with other building systems.

These documents can be used simultaneously to address construction phasing, inclusive of the needs of the project if the facility has to remain operational during the installation, as such requirements will have a direct impact on the cost and duration of the installation. Additionally, the bid documents should include an option for ongoing system maintenance and testing to meet local requirements.

Following distribution of the bid documents to qualified companies, a pre-bid meeting should be held with the potential bidders so they can understand the building and what is being proposed to be done, thereby setting the stage to conduct an apples-to-apples comparison of price to obtain the best overall value. Although characterized by a lower initial cost, an inferior installation will result in higher long-term maintenance and nuisance issues. The contactor's ability to perform the installation as well as the level of direct manufacturer support the contractor gets should be considered.

After contractor selection, it is important that the team identified at the beginning be kept in place to provide follow-up throughout the installation process. Inevitably, with a replacement project, unknown existing conditions will be discovered, and thus it is important that this team works with the contractor to ensure that cost-effective and functional solutions are developed. Through this type of cooperation, and with the benefit of clear documents to enhance communication and commissioning efforts, the many challenges of fire alarm retrofit and replacement can be effectively save money for the owner and yield a higher performance system in the long run.

 

 

Boat Lift Controller with Remote

Relay logic

Before describing the PLC programming, I’ll start by describing the type of relay logic I first considered.  This will lead right into the Ladder Logic design.

The first task in this design is to convert the momentary pulsed output of the radio receiver to a steady-on output to run the lift motor.  Also needed is a way to stop the motor when the lift reaches the full up or down position.  Since a relay is needed to safely switch the 110v rectified AC power, a standard solution to the design problem is to use a relay with an extra set of contacts to latch the relay on once it is activated.  In the diagram, the Up switch is a normally open pushbutton switch or, in the case of the radio controller I’m using, a transistor momentarily switched to ground.  When activated, current flows from the battery (or power supply) through the relay coil, the normally-closed limit switch, and the pushbutton or transistor to ground, activating the relay.  As the relay activates, the second set of contacts close, in parallel with the Up switch.  The Up switch can now be released and the relay and lift motor will stay activated.  (I have not illustrated the lift motor connections for simplicity.)  When the upper limit of lift travel is reached, the limit switch opens, breaking the circuit and stopping the motor.

 Simple relay design solution for lift motor control

This circuit will work, but it has many shortcomings:

      The only way to stop the lift is at the end of travel.  There is no way to stop it part way.

      I’d like to be able to stop and restart the lift by remote control, in a simple, intuitive way.  For example, push the Lock (up) button the remote a second time.

      For added safety, I’d like the lift to stop even if the Unlock (down) button is pushed.

      The limit switches are normally closed, meaning a short in the wiring would cause the lift to keep running.

The circuit can be modified to address the shortcomings, but it would get quite complex and require lots more relays.  I leave it as an exercise to design a complete circuit to address all the shortcomings.  I decided to move to a PLC in my design.

Ladder Logic

Let’s start by taking a look at a Ladder Logic design that mimics the basic control circuit.

Simple Ladder Logic design solution for lift motor control

The normally-open Up and normally closed Limit switches are represented by the vertical parallel lines, with a slash designating the normally-closed switch.  They are designated with a “I” for input.  The Run up relay coil is indicated at far right.  A set of contacts on this relay, which is used for latching the relay on, is over at the left.  It is labeled with the same name as the relay and designated with an “o” for output.  This Ladder Logic design can be downloaded into the PLC.  The PLC simulates what a corresponding set of relays and switches would do.

Can you see the similarity between the Ladder Logic diagram and the physical circuit schematic of the same design?  This is no coincidence.  It allows Ladder Logic to be understood by those used to working with relays without a lot of extra training.  There are many sources of Ladder Logic learning readily available on the web.  In fact, Triangle Research International offers a PLC simulator for their products, which can be downloaded free of charge.  I used the simulator to design my circuit before obtain the PLC hardware, to make sure that it would do the job.  As it happens, my design uses only about ¼ the capability of the E10-npn.

Bipes lift controller PLC program

Here’s my complete PLC program:

Complete Ladder Logic design solution for lift motor control

The first two rungs of the ladder correspond to the simple circuit with a few enhancements.  The middle rungs generate states necessary for the toggling action of the remote pushbuttons (push on/push of) that is desired.  The bottom rungs control the canopy lighting.  The bottom rungs use a feature offered by many PLCs: latching relays.  Since this construct is often used, this shorthand version is made available to the designer.  I could have used this construct in the upper rungs of my design as well.

Integrating Controls for Hydrogen Production

Hydrogen has long been discussed as a high potential alternative energy source, but most discussions of it focus on its use in automobiles. As such, the discussion quickly gets bogged down in two areas: infrastructure issues (replacing the current gasoline delivery pipeline) and the energy inputs required to produce hydrogen for fuel.

A description of the equipment used to produce and store hydrogen and deliver power at the Sotavento virtual power plant in Spain.

Hydrogen has long been discussed as a high potential alternative energy source, but most discussions of it focus on its use in automobiles. As such, the discussion quickly gets bogged down in two areas: infrastructure issues (replacing the current gasoline delivery pipeline) and the energy inputs required to produce hydrogen for fuel.

But there are multiple other uses for hydrogen as a fuel source, from powering buildings to electronics and equipment, and it can be created using the energy from alternative energy sources, not just fossil fuels. Hydrogen's important environmental advantage—when burned, hydrogen produces no contaminating emissions—has led to a growing interest in designing processes that use stored hydrogen as an energy source. Moreover, in cases where the hydrogen is produced by wind power or other renewable energy sources, the environmental impact is almost zero.
These considerations are behind an ongoing venture by the Sotavento Virtual Power Plant in Galicia, Spain, which was designed by Gas Natural SDG, Spain's largest energy company, in conjunction with the Galician Regional Government (Xunta) and the Sotavento Foundation. In essence, a virtual power plant is a group of distributed power generation installations operated collectively by a central control unit.
The project goals for Sotavento are to assess the suitability of hydrogen as a storable form of energy in its gas state. The company's objectives include:

• Commercial green energy production;
• Demonstration of the various wind technologies present in Galicia;
• Establishment of an education and training center;
• Establishment of a conference center for related events; and
• General promotion of renewable energy.

To do this effectively, however, integration of the control systems used to operate the various power generation units at Sotavento has to take place.
The system integration hurdle
For large enterprises like power plants and water treatment facilities, it is common for myriad disparate control systems to be in operation. In most cases, the control equipment—sourced from different vendors, each with its own area of expertise—is often composed of a programmable logic controller capable of operating a particular piece of equipment, but with little additional functionality. The operator interface to these controllers is typically some form of a local display panel with some or no connectivity to the rest of the enterprise.
To address Sotavento's multiple control system issues, Gas Natural contracted systems integrator Optomation Systems to design an integrated supervisory system for the Sotavento site. The system is based on the Opto 22 Snap PAC System platform (see sidebar box).
In consultation with Gas Natural, Optomation concluded that Sotavento's system would need to:

• Serve as a common manageable platform for process monitoring, data acquisition, and auxiliary control;
• Provide complete supervision of electrolyzers, compressors, and hydrogen motor-generator units via a common operator interface;
• Access and integrate data from the wind generator's existing SCADA system;
• Provide level and temperature monitoring in the hydrogen storage area (which is classified as explosive);
• Enable remote monitoring of the installation, along with remote stop/start/shutdown capability over a secure Internet connection;
• Provide data storage in a commercial relational database; and
• Export production data via the Internet.

Employing a mix of analog and digital I/O connections, Optomation used Snap PAC stand-alone and rack-mounted PACs to connect the electrolyzers, motor generator units, and other plant equipment and, as required, communicate with this equipment directly or via interface with other SCADA systems (see graphic).
Across the Sotavento sites, disparate machinery, systems, and instrumentation from vendors such as Hydrogenics, Emerson, and Bauer Compressors are all linked to the Opto 22 controllers, which communicate to each subsystem or machine using the same protocol originally specified by the manufacturer. Specifically, these protocols include Profibus, for control and acquisition of production data from the electrolyzer units, and Modbus for control and acquisition of production data from the compressors. There is also an RS-232 serial link to the motor-generator units for taking analog measurements and digital readings. The Snap PAC controllers aggregate all data and serve it to a Sotavento database that's accessible to select personnel via a secure Internet connection.

An outline of the various controllers, protocols, analog measurements, and digital I/O connected using Snap PAC across the Sotavento units.

Protocols and integrator expertise

"The secret to successful implementation of projects like this rests in defining the protocols at the hardware purchase stage, well before writing the first line of code," said Fabio Alberini, one of Optomation Systems' project managers on the Sotavento project. "If the customer fully understands and insists on the importance of data integration, suppliers will be obliged to include the necessary hardware interfaces and software support as part of their deliverables. Conversely, trying to design the communication links after the equipment is chosen and installed is more difficult, costly, and beyond the core competencies of the supplier."
Alberini advises control system purchasers to "understand what communications options are possible and always try to standardize on protocols." He maintains that there is still no better standard than Modbus for moving data between industrial devices. "It requires no special hardware or software interfaces," he says, "and it's royalty free and easy to implement. At the Ethernet level, Modbus/TCP is an even better alternative."
When hiring a system integrator, Alberini recommends ensuring that they have experience writing software for the hardware platform being used, as well as expertise integrating data.

Dynamic Simulation for Emissions Regulation

Assessing the impact of safety instrumented systems on column pressure relief load during plant upgrades or expansions.

Existing pressure relief and flare system capacity is sometimes challenged following unit upgrades and expansions, addition of new process units, or the re-routing of atmospheric vents to the existing flare system. Traditional relief load estimation methodologies are known to be overly conservative and can lead to the overdesign of flare systems or to the determination that such plant upgrades now require a flare capacity expansion.

In addressing these situations, dynamic simulation has become an accepted methodology to determine relief loads more accurately when traditional conservative methodologies indicate the existing flare is at or over capacity. Depew and Dessing reported significant reductions in peak relief load using dynamic simulation instead of traditional methods. However, the reductions from dynamic simulation are sometimes still not enough to offset the increased flare capacity requirements from the plant upgrade.

API 521 and AMSE Section VII Code Case 2211-1 provide an alternative to pressure relief devices, whereby a safety instrumented system (SIS) can be used to protect against over pressure. Traditional relief devices achieve pressure protection through controlled removal of the contents causing the over pressure, while the SIS approach focuses on removal of the cause of the over pressure itself. Since the SIS in this type of application involves substantial risk in the event of failure, it must be of high integrity and is thus often referred to as being a high integrity protection system (HIPS). Installing a HIPS can lead to large capital savings by eliminating the need for costly upgrades to an existing relief/flare system when even the dynamic simulation model indicates that additional flare capacity is needed.

   

This model was developed using Invensys' Dynsim software. The relief devices on the two towers discharge to a common flare header.

To illustrate the benefit of a HIPS, this article details a project performed to evaluate the peak relief rate for an integrated de-isobutanizer and de-butanizer, considering:

• Traditional unbalanced heat load approach;
• Rigorous dynamic simulation adhering to API 521 practices; and
• Rigorous dynamic simulation considering a HIPS on the re-boiler steam.

This type of analysis shows how the different approaches complement each other and how the model can be used to determine the appropriate set point of the HIPS system to avoid nuisance trips of the plant, while ensuring a significant reduction in relief load. This information can be used to compare the cost of installing and maintaining a HIPS versus expanding the flare system. Such a decision also needs to factor in the growing requirements of public and regulatory authorities to reduce flaring due to air quality and global warming concerns.

Dynamic simulation

Over the past decade, dynamic simulation has become a mature and recommended method for validating the design of chemical processes. Its benefits are cited in recent literature regarding the design of relief systems for complex distillation columns and the evaluation of compressor and other control systems. Benefits include:

• Greater accuracy in the calculation of column relief rates;
• Controls validation for optimal plant performance;
• Optimization of the normal start-up/shutdown procedures before plant commissioning; and
• Validation of operating strategies under abnormal conditions such as an emergency shutdown or trip.

Dynamic simulation can be applied to establish the effectiveness of HIPS to protect equipment and reduce the risk of a process exceeding its design limits. As the HIPS operates near the critical limit of a process and its integrity is vital to the safe operation of the plant, testing it on the plant can involve a great deal of risk. A dynamic simulation study can be used effectively to help validate the effectiveness of the HIPS by simulating its behavior, safely, on a computer.

This graphic shows the non-linear variation of peak relief load changing with the HIPS set point. In this particular case, the dynamic simulation model was configured to run the governing scenarios repeatedly with changing set points to come up with an optimum value for the set point.

Peak relief load calculations

The use of dynamic simulation for the analysis of HIPS can be further extended to evaluating HIPS on fired heater re-boilers and furnaces where the impact of residual heat capacitance in these equipments can be modeled to study their impact on flare loads. It can also be used to understand the behavior of extremely exothermic reactors where faster pressure and temperature transients become critical and where traditional relief devices may not work properly.

The application detailed in this article involved the use of a dynamic simulation study to predict the relief load of an integrated de-isobutanizer tower and a debutanizer tower in the alkylation unit of a refinery without any safety instrumentation and with a HIPS that cut off the supply of steam to the column re-boilers when the pressures in the columns reached a pre-determined set-point.

The de-isobutanizer and debutanizer towers each produce a distillate and a bottoms product, with each tower's overhead vapor passing through a single drum overhead system and each having a single thermo-siphon steam heated re-boiler. The condensing duty on each tower is provided by cold water. The liquid feed to the de-isobutanizer primarily consists of i-butane, butane, and heaviers, while the vapor feed is a mixture of butane and i-butane. The feed to the de-butanizer is the bottoms of the de-isobutanizer. This flow is pressure driven since the de-isobutanizer operates at a higher pressure and elevation.

Two scenarios were tested on the dynamic simulation model to estimate the peak relief loads:

1. Total power failure: Causes the feed to trip, the electrically driven pumps to trip and loss of condensing duty. However, the supply of steam to the reboilers is assumed to continue.
2. Loss of cooling water: Causes loss of condensing duty while maintaining the feed into the towers and steam to the re-boilers.

As per API 521 recommendations, the simulation model was run for 30 minutes beyond the start of the upset. As shown in the "Dynamic simulation study results" graphic, the total peak relief rate determined by the rigorous dynamic simulation was 20% lower than the values estimated using the conventional unbalanced heat load calculations. The results also show that the HIPS system substantially reduced the peak relief loads for these columns and even eliminated the relief load from the de-isobutanizer.

Determination of the HIPS set point was also an important part of the solution delivered to the client. Setting it too high could have led to there not being a significant reduction in the relief load; setting it too low could have led to an increased rate of nuisance trips of the re-boiler when there are normal disturbances to the process. The response of the combined relief rate from both the towers to changing HIPS set points is not a linear problem with an easy solution.

Outcomes

This work was critical to helping plant management eliminate the need for an expensive re-design of the flare piping network for this unit. Dynamic simulation can also be used to model more complex and integrated processing units to determine the optimum configuration of multiple HIPS for safety shut down systems or for pressure relief load reduction as implemented here.

The dynamic simulation models were also effectively used to identify the optimum set point for the HIPS on this fairly straightforward unit. For more complex integrated processing units where equipments and units interact with each other, determination of optimum HIPS set points can be a difficult challenge. In such cases, dynamic modeling of the process and careful sensitivity analysis can be used to help make this determination.

It is also worth noting that the decision to proceed with using safety instrumented systems requires careful examination of applicable regulations and standards. These may include local body regulations and insurer's requirements.

Dynamic simulation study results

	Cooling water failure			Total power failure
	Conventional method	Dynamic simulation	Dynamic simulation with HIPS	Conventional method	Dynamic simulation	Dynamic simulation with HIPS
De-isobutanizer	519,860	431,380	0	386,956	37,720	0
De-butanizer	114,080	76,475	24,350	207,000	9,875	0
Peak flows shown in this table are calculated in kg/hr.

   

Author Information

Abhilash Nair is principal consultant, and Ian Willetts is director of process modeling and simulation, Invensys Operations Management, Plano, TX.

Alan Wade, is a faculty member of the Department of Engineering Science, University of Oxford, England.

Cellular Gateway

1.0. Overview

Historically, monitoring and control of remote sites required very little data bandwidth. Remote sites typically

had a few I/O and communicated over voice-grade phone lines. After the advent of PLCs and RTUs, many

remote operations had to improve communications to go along with the upgrade to these digital devices.

However, even PLCs and RTUs sometimes do not have enough data bandwidth to perform as needed.

Today, operators are asking for more capabilities in their remote operations, such as Video Surveillance and

remote Access Control.

In a modern control system with high speed networks, video surveillance for security, process control and

automation purposes can use the existing plant network to interface with Supervisory Control and Data

Acquisition (SCADA) systems and HMIs, such as Wonderware, Rockwell and GE Fanuc. However, many

control systems today do not utilize high speed networks and still communicate via slower connections.

While some video systems, such as the

Longwatch Video System, are designed

for low bandwidth networks, there are still

many remote sites, Figure 1, that have insufficient

infrastructure, obsolete technologies

or high levels of proprietary technology,

making it difficult for even Longwatch

to operate effectively. These include DC

telephone circuits, tone (FSK) communications,

proprietary radio networks, extremely

slow networks (300 bps), and others.

There is a very short and expensive list of

upgrade paths that will result in a network

that will be sufficient for video surveillance

applications. While all upgrade options

will require some level of engineering, the

simplest and fastest upgrade is to install a Cellular Gateway. With data rates up to 2 Mbps, a cellular connection

can handle most remote site communications needs.

The term Cellular Gateway refers to a device that acts as an interface between a control room computer or

LAN and a remote site through a cellular data connection. These gateways can provide high performance

wireless TCP/IP data communications via cellular networks for connecting remote sites and devices. This

communication pathway is secure and “always on,” allowing for on-demand transfer of data to both Ethernet

and/or serial devices. Setting up a cellular gateway is often simpler and much less expensive than installing

a point-to-point wireless system. Table 1 illustrates the main steps in configuring a Longwatch video

system via a cellular gateway.