# Parsing pressure and temperature ratings

Oct. 9, 2019
Will a fail-closed valve open when the downstream pressure exceeds the upstream pressure?

This column is moderated by Béla Lipták, automation and safety consultant and editor of the Instrument and Automation Engineers' Handbook (IAEH). If you have an automation-related question for this column, write to [email protected].

Q: What is the difference between maximum allowable working pressure/temperature, maximum operating pressure/temperature, maximum allowable operating pressure/temperature, design pressure/temperature and storage pressure/temperature?

Mehdi Manouchehri

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A: Following is an extract from "Guidelines on Dp, Op, Mop, Maop, Mawp, Psv Setpoints," available at several web sites. I hope it clarifies correctly:

Operating pressure (OP): This is the pressure at which the device/equipment/pressure vessel is operating under normal conditions. Simply put, you could say the general pressure conditions in the equipment on a regular basis.

Maximum operating pressure (MOP): The maximum operating pressure that the engineer considers will be encountered in the process operation, including a margin for any possible surges or fluctuations.

Design pressure (DP): Per ASME B31.3, this is the most severe and coincident condition, internal or external, due to the service that the equipment is likely to be subjected, meaning the worst possible temperature and pressure case the fluid can exert on the vessel/equipment. This is decided by the engineer based on the fluid service existing, and it also varies with the fluid. The engineer decides it based on previous experience and a sound judgment.

Maximum allowable operating pressure (MAOP): MAOP is not to be confused with the MOP. It is a definition mainly formulated for pressurized equipment such as cylinders or pressure vessels, but also applicable elsewhere. It is considered the maximum possible safe pressure that the walls of the equipment can withstand.

Maximum allowable working pressure (MAWP): This is defined as the maximum pressure experienced by the weakest point of the equipment before failure. Some mistakenly refer to it as design pressure but simply put, it tells us the maximum pressure that the weakest point in the equipment can withstand before becoming unpredictable or failing (also referred to as design pressure of the weakest point, but not the general design pressure as such). An alternative and equally valid definition for pressure vessels is, it is the maximum permissible pressure at the highest point or top of the vessel in normal operating conditions at the coincident operating temperature for that pressure. The MAWP is usually determined by the manufacturer of the equipment based on fabrication procedures, backward correlations, etc. and is usually stamped on the equipment.

The increasing order of the above values is: OP < MOP < DP < MAOP < MAWP. None of other values are supposed to exceed the MAWP. The MAWP is usually about 10-25% above the operating pressure, but not always.

Alex (Alejandro) Varga
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A: Maximum allowable working pressure is used for sizing relief valves. It’s 10% above the pressure where the relief valve is actuated. Maximum operating pressure/temperature is the maximum operating condition. Maximum allowable operating pressure is the same as the maximum allowable working pressure.

Design pressure/temperature are the pressure/temperature at which equipment is designed to normally operate. There is a maximum pressure also, beyond which equipment integrity is compromised. There is also a spike that it can tolerate for a few seconds.

Storage pressure/temperature is for the product to be stored at.

Hiten A. Dalal, P.E.
Senior automation engineer, pipeline products
[email protected]

A: The maximum allowable working pressure/temperature (MAWP) is the pressure that the pipe and components will be subjected to before a protective function kicks in (e.g., 850 kPag for a 700 kPag gas pipeline. 850 kPag is the setpoint for the slam-shut valve that protects the downstream equipment).

Maximum operating pressure/temperature: Say the operating pressure range is 700-770 kPag (active rgulator is set at 700 kPag, monitor regulator is set at 750 kPag). Hence the maximimum operating pressure is 770 kPag.

Maximum allowable operating pressure/temperature: The maximum allowable operating pressure is determined and set by the end user. The manufacturer/supplier then selects equipment that will operate above this pressure, but incorporates protective functions to limit the operating pressure (see above).

Design pressure/temperature: Design pressure is the guaranteed safe pressure for the equipment (in this case, the pipe). Usually, the design pressure will be selected as 1,000 kPag to allow for thermal expansion, and the weep relief valve will be set at this pressure.

Storage pressure/temperature: Storage temperature is the maximum temperature at which the purchased equipment is stored in the warehouse. It could be, say, 50 °C during very hot summer when power is not available.

M. Binney
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Q: Will a fail-closed (FC), double-acting valve open if downstream pressure exceeds upstream? We have one double-acting, air-actuated, FC angle valve installed where the upstream pressure is 70 kg/cm2 and downstream pressure is 14 kg/cm2. The control valve has one volume tank that is feeding air from top of the actuator through an air lock valve to keep the valve closed in case the air supply to the positioner fails.

We planned an air filter/regulator (AFR) replacement job for the volume tank. The operator isolated the downstream isolation valve and said to start the job. The main air supply was isolated and the air drained from AFR of main air line to the positioner. The control valve remained in closed position. Then the volume tank air was drained, and as the pressure dropped to 1 kg/cm2, the angle valve (air fail to close) suddenly opened. The procedure was repeated and the valve opened again. I mention that all the tubing connections are ok, as was the and vent and air lock relay venting the air. After the AFR replacement job was completed and air was normalized, the control valve closed again. The downstream isolation valve was opened.

The next day, we tried the same thing to check the fail safe action of control valve, but this time the upstream isolation valve was isolated instead of the downstream isolation valve. When the main air was failed, the control valve remained in closed position and when the volume tank air was drained, the valve remained in the closed position.

Please confirm my thought that in the first case the control valve opened because the downstream isolation valve was closed and the back pressure forced the plug to open when the air pressure in the VT dropped to less than 1 kg/cm2 to zero.

I am not finding this exact situation in any catalog or manual. The make of the angle valve is CCI-Drag.

Bhartendu Nayak
[email protected]

A: The design of a CCI Drag valve is such that the process fluid pushes the valve plug toward the open position, as is the case with most control valves. That means that if all motive air is removed from both sides of the actuator, but the valve is left pressured with process fluid, the valve will open regardless of its failure mode. To be safe, you must close its upstream block valve and depressure the entire piping in which the valve is installed. That probably means to close the discharge block vale as well. That is why the valve has isolation valves and a low point drain in the piping. (I hope!)

This practice must be followed when servicing any kind of valve. Do not depend on any claimed failure mode. It may be a nuisance to do all this work but it is your neck that is on the line.

Walter Driedger.
[email protected]

A: On a double-acting-type actuator, fail safe mode, to maintain air pressure to the actuator was the responsibility of the pilot valves and air reservoir. In case of power and pressure system failure, the pilot valve will move to its failure position so air from the reservoir flows to actuator to maintain the valve in the closed position. If there is no pressure loss in this system (due to leaks on tube fitting, reservoir or failure of the pilot valve), the control valve will remain closed with no power and no pressure (failure of power and pressure system, e.g., plant blackout scenario).
On the control valve data sheet, check the maximum delta pressure (DP) in the closed-valve position. If the DP of the valve at those conditions did not exceed the maximum DP across the valve stated on valve data sheet, it's impossible to say that the control valve opened due to excessive upstream/downstream pressure.

In my experience, control valve manufacturers always ensure that their actuator has enough power to keep the valve at closed position at maximum delta pressure. I've suspected that the disturbances of air supply of control valve (actuator) fail-safe system might cause the valve to suddenly open.

Budi Yuwonob
[email protected]

### Béla Lipták | Columnist and Control Consultant

Béla Lipták is an automation and safety consultant and editor of the Instrument and Automation Engineers’ Handbook (IAEH).

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