Catch it to me gently,
Feets don't fail me now (don't fail me)
Please, don't fail me now (don't fail me)

("Feets Don't Fail Me Now," by Little Feat, 1974)

 

VALVE FUNCTION #4:
VALVES THAT SAFELY MANAGE AND GET RID OF BUILT-UP PRESSURE

The fourth and final job of Valves is to respond to increases in the PV Pressure that would damage equipment ... and much worse ... kill people.

Both the Pressure Relief Valve (PRV) and the Pressure Safety Valve (PSV) have the job of returning an over-pressured process system to a safe operating range of Pressure.

PTOA Readers and Students will notice many structural similarities between PRVs and PSVs. Even the ISA Symbol for a PSV and a PRV is the same!

The difference between the PRV and the PSV is their job descriptions.

The PRV limits the PV Pressure that builds up in a compressed air or liquid process stream by routinely relieving the excess Pressure. Ergo, the PRV protects equipment from the wear and tear that would be caused by operating in a sustained over-pressured environment.

The PSV protects equipment from catastrophic failure and ... much more importantly ... the PSV saves lives and typically those lives are the lives of Process Operators.

The PSV performs its job by rapidly removing Pressure that has suddenly built up in gases or vapors (like Steam) while these fluids are contained in piping, Vessels, Towers, Reactors, Shell and Tube Exchangers, etc.

What type of incidents can cause a process system to overpressure? Here's a few possibilities:

• 

  Note the PSV in this P&ID snippet is set to pop at 900 psig. The piping around a PSV is intentional and purposeful. The 4-inch inlet process line is reduced to a 3-inch diameter, thus a higher velocity process stream is targeted on the bottom of the valve. Note the outlet process line is 6 inches in diameter and is thence reduced to 4 inches.

  A Process Operator has started up a pump and the Discharge Valve is still blocked in.

• A Process Operator has opened up a Valve under much greater process pressure than the line pressure on the other side of the Valve.
• A Control Board Operator loses control of the reactions taking place in an exothermic Reactor (featured in PTOA #28 and PTOA Segment #37.

• 

  This is a graphic of the top of a Vessel. The PRV protects the Vessel from over-pressure.

  A failure of the electricity utility has occurred.

• A failure of the instrument air utility has occurred.
• A failure in cooling equipment has occurred; For example, the Cooling Water System featured in PTOA Segments #39 through PTOA Segment #41 has an outage or the Fin Fan Condenser featured in PTOA Segment #38 has corroded tubes or a fan failure.
• 

  Four souls were killed and the facility in ruins after a runaway chemical reaction.

  A failure of a Shell and Tube Heat Exchangers like those featured in PTOA Segment #30 through PTOA Segment #35 due to stress corrosion cracking, vibration or thermal shock.

• A process unit fire caused by welding, electrical arcing, bad housekeeping practices, etc.

 

Note that the list of root causes can be effectively reduced with well trained, alert Process Operators and a management team committed to preventative maintenance.

 

 

PRESSURE RELIEF VALVES (PRVS)

Pressure Relief Valves (PRVs) protect processing equipment from working in an over-pressured processing environment.

Beware! The Pressure Relief Valve (PRV) does not have the same job as a Pressure REDUCING Valve. Pressure Reducing Valves are made from throttling-type Valves and their job and positioning is to intentionally reduce the PV Pressure of the flowing process stream.

 

Function of PRVs

The Pressure Relief Valve (PRV) is the first line of defense to protect equipment from an over pressuring event and thus avoid inconvenient and costly downtime.

Primarily used in upgrading liquid process streams and in compressed air generation, the PRV maintains operations around a preset Setpoint Pressure.

The PRV starts opening when the PV Pressure of the flowing fluid attains 10% of the maximum allowable working pressure (of the lowest pressure-rated component in the process system). The PRV will be fully open should the sensed line PV Pressure increase to a magnitude that is 20% over-Pressure.

 

Form of PRVs

The nearby graphic features a cutaway of a PRV and includes a corresponding diagram with labelled components.

The force of the Spring is bearing down upon the upper surface of the Disc. The Inlet Nozzle directs the velocity of the process stream to the bottom surface of the Disc.

In the closed position, the upward force factor of the process stream's Pressure is balanced with the downward force of the Spring.

The Disc will remain in the closed position until the process stream's PV Pressure exceeds a pre-determined Setpoint. When the Setpoint PV Pressure is reached, the force factor of the process stream's PV Pressure is greater than the downward force of the Spring which causes the PRV to open.

The Setpoint is manually set by the Adjusting Screw. The Setpoint will be within 10% of the maximum allowable working Pressure of the hardware in the process system.

Building up the PV Pressure in a liquid process stream does not happen fast.  Thus, the Disc will move gradually once the force of the process stream exceeds the force of the downward Spring. The exiting process stream enters the Outlet pipe diameter that is twice as big as the Inlet pipe diameter.

Once the built-up PV Pressure in the process stream reduces below the Setpoint, the Disc will return to the closed position.

The cycle will repeat should the built-up flowing PV Pressure of the liquid process stream (or compressed air) again reach within 10% of the maximum allowable working Pressure of the hardware that is being protected by the PRV.

Because of their many physical similarities, even seasoned Process Operators get confused distinguishing PRVs from PSVs.

Here's another difference between PRVs and PSVs:

The over-pressured liquid exiting the PRV flows to a different place in the system that is at a lower PV Pressure and eventually will be returned to the process.

 

The over-pressured gas or vapor rapidly exiting a PSV is completely removed from the system.

The nearby AMJ Youtube illustrates how the over-pressured liquid at a pump's discharge is sent back to the liquid's supply tank. PTOA Readers and Students do not need to stress about the PRV design details featured in the AMJ Youtube.  The video illustrates how the PRV opens gradually, based upon a percentage of PV Pressure ... and then reseats after the PV Pressure is once again below Setpoint.

The video also illustrates the operating and process service differences between direct-acting PRVs and pilot-operated PRVs.

 

PRESSURE SAFETY VALVES (PSVs)

Pressure Safety Valves (PSV) are the last resort option to prevent catastrophic loss of life and equipment.  

The list of root causes that result in over-pressuring a process appeared earlier in this PTOA Segment.

Which brilliant PTOA Readers and Students noticed the predominance of over-pressure root causes related to the inability to cool a process stream?

Why is the ability to cool a gaseous flow critically important?

Why Gases Suddenly Build-Up Pressure: Compressibility

Brilliant PTOA Readers and Students already know that gases are compressible, and liquids are not (see PTOA Segment #153).

Here's a quickie review about the meaning of "Compressibility":

A gas is compressible because a decrease in Volume will immediately double the Pressure of the gas, and vice versa.

Okay ... so what is the tie between loss of cooling and a gas rapidly building up pressure?

A sudden increase in the PV Pressure of a gas is typically caused by an unnoticed or otherwise unexpected increase in the PV Temperature.

PTOA Readers and Students learned about "Gas Laws" in PTOA Segments #152 through #154. As gas molecules are heated, they become more and more agitated and bang into each other and seek a greater Volume to occupy.

Ergo, when a gas is heated, the only way to maintain the same Pressure is for the Volume of the container that the gas is in to expand simultaneously.

The rigid walls of the process facility hardware cannot expand!

The PV Pressure rapidly builds up in a gas that is heated. The PV Pressure of a heated gas will eventually exceed the maximum working pressure of the hardware in which the gas in contained. Then ... Boom! 

Typically, the over-pressuring rupture will occur at a flanged connection (see PTOA Segment #250).

 

Function of PSVs

To do their job of preventing equipment rupture and loss of life, PSVs open suddenly and completely which is totally different than the gradual opening of Pressure Relief Valves (PRVs).

Once more from the PTOA Department of Redundancy Department:

The Job of the Process SAFETY Valve is to react to a sudden over pressuring of gases or vapors (like Steam) by rapidly opening 100%, thus enabling the removal of excessively-pressured gases and vapors away from the process.

The PSV will start opening when the PV Pressure of the flowing process gas increases to 3% of the maximum allowable working Pressure and then rapidly opens to 100% open.

The suddenly exiting flow of gas will be accompanied by unmistakable loud noise. Shortly thereafter, the flames of the Flare will shoot high into the sky.

A burning flare looks really scary, Fred ... but a burning Flare means that lives have been protected.

PTOA Readers and Students need to visualize the process plant situation that is being described:

The PSV only pops when the PV Pressure has suddenly built up so fast that the unit needs to be automatically depressured rapidly.  Once the PSV pops open, an unexpected Emergency Shutdown is happening.

As was described above, the discharged fluid from a Pressure Safety Valve flows into the Flare Utility System. The ejected gases and light hydrocarbons will be burned in the Flare, hence released as carbon dioxide and water into the ambient atmosphere.

The PSV will not start closing until the system Pressure has reduced to a specified Reset Pressure.

 

Form of a PSV

A cutaway diagram of a PSV is nearby. The hardware of the PRV and PSV have many similarities. The noteworthy differences include the absence of an Adjusting Screw because Process Operators do not adjust a PSV.

Making certain a PSV is in condition to do its job protecting lives and equipment is the job of Valve specialists, not Process Operators.

After a Pressure Safety Valve has done its job and 'popped off' it will need to be certified to be put back in service. Reset of the PSV may very well cause a delay in the process unit Start Up that follows the emergency Shutdown. Waiting for the PSV to be certified for service is worth the wait! The lives of Process Operators and other plant personnel depend upon the PSV working correctly!

 

TAKE HOME MESSAGES: The fourth job of Valves is to protect equipment and save lives by preventing the buildup of excessive PV Pressure. 

Pressure Relief Valves (PRVs) and Pressure Safety Valves (PSVs) have physical similarities and share the same ISA symbol. The PRV and PSV differ in their job description and process service, the manner in which they open and close, where they direct the exiting process stream, and the existence or lack of an Adjusting Screw that determines the Setpoint Pressure.

PRVs and PSVs have different process stream services:

• PRVs manage the PV Pressure of liquids and compressed air systems.
• PSVs immediately get rid and remove excessive PV Pressure that has built up in gases or vapors (like Steam).

 

PRVs and PSV have different jobs:

• PRVs maintain the PV Pressure around an adjustable Setpoint Pressure, thus preventing the equipment from operating in a sustained over-pressured environment. Once the PV Pressure reaches the Setpoint, the PRV starts to open and gradually reduces the PV Pressure which protects equipment. The Valve closes once the PV Pressure is reduced below the Setpoint Pressure ... and the cycle of equipment protection continues. 
• PSVs immediately and completely remove excess PV Pressure from the processing system, thus the PSV prevents a catastrophic event from destroying property and killing people. The PSV opens once the PV Pressure is sensed to be within 3% of maximum allowable working pressure and rapidly opens up to 100%. Once the PSV has "popped," an Emergency Shutdown occurs.

 

PRVs and PSVs deal with the exiting process fluid differently:

• The liquid process stream exiting the PRV is sent to a place in the system with lower PV Pressure; the liquid is not discarded.
• The gas or vapor exiting a PSV is immediately removed from the processing facility, typically sent to a Flare System where it is incinerated.

 

PRVs and PSVs have different procedures for Setpoint adjustment/certification: 

• The PRV has an adjusting screw that the Process Operator can adjust if necessary to change the PRV's Setpoint PV Pressure.
• The PSV does not have an adjusting screw and can only be certified for service by a professional Valve specialist after it has popped off.

 

Pressure Relief Valves (PRVs) should not be confused with Pressure Reducing Valves. Pressure Reducing Valves are Globe Valves with the job of intentionally causing a Pressure Drop in the flowing fluid; the PV Pressure sensed at the outlet of the Pressure Reducing Valve is significantly less than the PV Pressure sensed at the inlet of the Pressure Reducing Valve.  

 

©2024 PTOA Segment 0255

PTOA PV FLOWRATE FOCUS STUDY AREA
PIPING NETWORK HARDWARE

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