Pressure Control And Relief

The first line of defense in process safety regarding pressure is to keep the operating pressure within safe limits. The inherently safe way to do this would be to design a process so that pressure never exceeds the maximum allowable working pressure (MAWP) of any of the equipment involved. When this cannot be achieved, pressure control and relief systems must be used to keep process pressure from increasing to a dangerous level. These systems, such as the one in the block diagram, typically have a user-defined set-point pressure signal that is sent to a pressure controller, which compares it with the signal generated by a pressure transmitter, which measures the actual pressure of the process. After some control calculations, the controller sends a control signal to the pressure control element in the process, which is typically a control valve, and the valve opening changes so that the pressure can better match the set-point. In cases where pressure control systems are not in place, or when the control system fails, pressure relief devices are used to lower process pressure. When control or relief valves allow liquids or gases to exit process equipment to maintain a safe operating pressure, this fluid is sent to secondary containment equipment.

Case Study

In 2008, a heat exchanger rupture and ammonia release at a Goodyear Tire and Rubber Company plant killed one employee and injured six others. This incident happened when an isolation valve placed in between the heat exchanger and relief valve was closed in order to replace a burst rupture disc. The next day, a block valve between the heat exchanger and a pressure control valve was closed, and steam was supplied to the process in order to clean out the inside of the piping system. When steam was sent through the heat exchanger, it heated the ammonia in the shell, increasing the pressure within the shell. This pressure could not be relieved since the block and isolation valves were closed. The shell ruptured, killing one worker and injuring six more with the release of ammonia. The picture below was taken after the heat exchanger ruptured, showing the damage to the equipment and surrounding area. This case study demonstrates the importance of always having an open path to relief devices, even when maintenance is being performed. For more case studies, please visit www.csb.gov .

(Courtesy of the U.S. Chemical Safety Board)

Pressure Control Equipment

A pressure control system consists of transmitters, controllers, and control elements, which are typically control valves and actuators.

Transmitters

Pressure transmitters, such as the one pictured below to the left, are common in industrial pressure control systems. These devices contain a pressure sensor element which measures pressure electronically by converting a force or displacement caused by the pressure in a given process into an electrical signal. This signal is sent to a controller, which compares the signal to a set-point value, and determines whether to activate the pressure control element in the process. Some common pressure sensor elements include strain gauges, capacitors, piezoelectric elements, and linear variable differential transformers (LVDTs).

(Courtesy of Emerson Process Management, Chanhassen, MN)

(Courtesy of Emerson Process Management, Chanhassen, MN)

, as pictured above to the right, typically consist of wires connected to a frame or plate, which stretch and contract to reflect the pressure applied. These devices are categorized as being bonded or unbonded.

Transmitters are common in industrial applications because they contain a built-in output amplifier, unlike transducers which do not have an output amplifier and are mainly used in a laboratory setting. Transmitters often have reset and calibration options, which are not typical of transducers. For more information , please see the pressure measurement module.

Controllers

Pressure controllers, pictured below, receive signals from transmitters and compare them to set-point signals. Controllers use control calculations to create an output signal which is sent to control elements which can be analog or digital. Digital controllers often require analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) to change between each type of signal depending on the compatibility of the transmitters, controllers, and control elements. Controllers come in many control modes, including proportional, integral, derivative, and proportional-integral-derivative or PID control.

(Courtesy of Emerson Process Management, Chanhassen, MN)

Control Elements

Control elements in pressure control systems are typically control valves, which are opened and closed by actuators.

Control Valves

Control valves are the final control element in many pressure control systems. These devices open and close based on an output signal sent from a controller. The signal is usually sent to the actuator, which controls a valve it is attached to. Valve bodies used for pressure control valves include: (pictured below, to the left), (pictured below, to the right), , and .

(Courtesy of Emerson Process Management, Chanhassen, MN)

(Courtesy of Emerson Process Management, Chanhassen, MN)

Actuators

Actuators are used to control valves. Actuators can be pneumatic, meaning they use pressurized air to regulate valve opening and closing, or electric, which use electric power for regulation.

The pneumatic diaphragm actuator, shown on the left below, is attached to the top of a valve. When pressure below the diaphragm of this actuator becomes greater than the pressure above it, the diaphragm will move upward and the valve will open. To the right of the pneumatic diaphragm actuator is an electro-mechanical actuator, which controls valve opening by receiving an electrical signal which activates a motorized gear that changes valve opening position.

(Copyright Valtorc International, Kennesaw, GA)

(Copyright Columbus McKinnon Corporation, Amherst, NY)

Pressure Relief Equipment

Even if a pressure control system is in place, a pressure relief system is needed in any process that has the potential to generate high pressures, in case the control system fails. It is also important to have redundancies in a pressure relief system so that any individual equipment failures do not compromise the safety of a given process. Common relief devices are pressure relief valves and rupture discs.

(Copyright Chemical Engineering, Access Intelligence, LLC)

Relief Devices

in pressure relief systems open at a preset pressure so that excess pressure and the contents causing that excess pressure can be relieved from a process. There are three main types of valves: relief valves, safety valves , and safety relief valves. Relief valves are for liquid relief only, while safety valves work for vapor, gas, and steam. Both types of valves open once the operating pressure exceeds the set pressure of the valve and close once the pressure approaches the set pressure again. Safety relief valves function as either relief or safety valves depending on the phase of the fluid present in the process.

Pressure relief valves play an important role in pressure relief systems since they allow some of the fluid from the high pressure region to be relieved from the location of increased operating pressure to be relieved before any harm can be done to the process or any people in close proximity. Without these valves in place, excess pressure could build up to the point of pressure vessel or pipeline failures that have the potential to release toxic and flammable substances from the process and into the surrounding area.

One very important aspect of pressure relief devices is how they are sized. The size of a pressure relief device is important because it needs to possess an adequate discharge surface area to relief fluid with, and the size will also help determine the piping diameter at the inlet and outlet of the device. If a relief is over or undersized, equipment failure is likely to occur.

(Courtesy of Emerson Process Management, Chanhassen, MN)

Isolation valves prevent fluid flow to a particular location. This is often required so that maintenance can be performed in safe conditions. Many types of valves that have fully off and on configuration can be used as isolation valves, including: , , , , , and . These valves are needed when multiple pressure relief valves are installed in order to isolate one valve for maintenance while also being able to use the other valve when necessary. It is very important to remember that there needs to be an open path to an overpressure relief device even when maintenance on equipment is being performed.

Bleeder valves are used to lower the pressure of a section of piping. These valves are often needed in between pressure relief valves and isolation valves to lower pressure in between the other two valves for maintenance safety purposes. Most bleeder valves are either or valves.

(Copyright Chemical Engineering, Access Intelligence, LLC)

Rupture discs are devices that are designed to burst open at a pressure that is specified by the material properties and construction of the disc. They are often used by themselves when the relief line is to be left open after the disc ruptures. These devices can be placed in series with spring-loaded reliefs to decrease exposure of relief devices to a corrosive substance, and also isolate toxic or flammable materials.

Rupture discs enhance process safety by relieving fluids and lowering operating pressure, protecting other equipment from corrosive and toxic chemicals, and informing process personnel of the pressure value at the rupture of the particular disc. These discs can be placed in series with other relief devices as a form of protection so that the other equipment will be isolated from harmful substances that may lower their effectiveness and service life. Also, rupture discs can be placed in series with other rupture discs to help indicate what pressure is being exhibited by the system.

(Courtesy of Continental Disc Corporation | Groth Corporation | LaMOT Brand Products.)

Steam traps release air, condensate, and noncondensable gases from a pressurized stream. Some typical steam trap designs include: inverted bucket, shown below, float and thermostatic, thermodynamic, and thermostatic.

(Copyright Armstrong International, Three Rivers, MI)

Secondary Containment Equipment

Secondary containment typically starts with fluid relief at the location of a pressure relief valve or a rupture disc. The fluid then travels through outlet piping and is treated to leave the process in a safe form.

Knockout drums are that are used when the relieved fluid contains a considerable amount of liquid that is usually hazardous or corrosive. These vessels are also necessary when vapor cannot be routed directly to a main flare header in preparation for combustion in a flare stack. The picture below illustrates how a knockout drum separates vapor and liquid phases, and temporarily stores the liquid separated.

Blowdown tanks are typically used to collect the liquid that is temporarily stored in the knockout drums. If the collected liquid has a tendency to solidify then the blowdown tank may need a heat source to prevent congealing.

, or scrubbers, are used to clean gas streams before they exit a process. Scrubbers use a liquid to absorb the contaminants in the gas as they pass through it. Some of the common types of scrubbers include packed-bed, tray, and bubble columns, venturi, and wet scrubbers.

(Copyright Tri-Mer Corporation, Owosso, MI)

or flare stacks, are designed to eliminate toxic and flammable vapors that have been relieved from a process. These stacks convert the hazardous vapors into safe products that can be released from the process. Flares are often categorized as either elevated or ground flares.

Flare headers are piping systems that relieved fluids pass through to undergo combustion in the flare stack. These systems may send fluid directly to the flare stack, or they may send fluid to knockout drums which separate the vapor-liquid mixtures so that vapor can reach the flare stack.

(Copyright FLAREGAS Corporation, Nanuet, NY)

Osha Regulations

Standard 1910 Subpart H presents the standards for handling hazardous gases and equipment present in pressure control and relief applications.

Acknowledgements

Armstrong International, Inc. , Three Rivers, MI

Chemical Engineering , Access Intelligence, LLC

Chemical Engineering Department, University of Michigan, Ann Arbor, MI

U.S. Chemical Safety Board

Continental Disc Corporation

Emerson Process Management

FLAREGAS Corporation , Nanuet, NY

MKS Instruments, Andover, MA

Swagelok Company

Tri-Mer Corporation , Owosso, MI

References

Cross, James, personal communication, 2016.

Crowl, Daniel A., Louvar, Joseph F. Chemical Process Safety: Fundamentals with Applications &nbspNew Jersey: Prentice Hall P T R, 1990. Print.

Crowl, Daniel A., and Scott A. Tipler. "Sizing Pressure-Relief Devices." Sizing Pressure-Relief     &nbspDevices. AICHE, Oct. 2013. Web. 30 Nov. 2015.

Doyle III, Francis J., Edgar, Thomas F., Mellichamp, Duncan A., and Seborg, Dale E. Process     &nbspDynamics and Control . 3rd ed. New York: John Wiley & Sons, 2011. Print.

Guidelines for Pressure Relief and Effluent Handling Systems . New York, NY: Institute, 1998.     &nbspWeb.

Mukherjee, Siddhartha. Pressure-Relief System Design . Chemical Engineering, November     &nbsp2008: 40 - 45.

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