Valves sizing

Most relief valve manufacturers have software available for sizing relief valves. These programs are relatively easy to use. Understanding how relief valves are sized and inputting the correct information into the program are essential for calculating correct sizes. 

The flow of a compressible fluid through an orifice is limited by critical flow. Critical flow is also referred to as choked flow, sonic flow, or Mach 1. It can occur at a restriction in a line such as a relief valve orifice or a choke, where piping goes from a small branch into a larger header, where pipe size increases, or at the vent tip. The maximum flow occurs at 

Pcf - critical flow outlet pressure, psia Pi = inlet relieving pressure, psia 

For gases with specific heat ratios of approximately 1.4, the critical pressure ratio is approximately 0.5. For hydrocarbon service, this means that if the back-pressure on the relief valve is greater than 50% of the set pressure, then the capacity of the valve will be reduced. In other words, if the pressure in the relief piping at the valve outlet is greater than half (he set pressure, then a larger relief valve will be required to handle the same amount of fluid. 

As long as the pressure ratio exceeds the critical-pressure ratio, the throughput will vary with the inlet pressure and be independent of outlet pressure. For example, a relief valve set at 100 psi will have the same gas flow through it as long as the back-pressure is less than approximately 50 psi. 

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Nominal valve size Class 300 steel Class 400 steel Class 600 steel ... 

Nominal valve size Gate Globe, lift check, swing check A and B Angle and lift check B... 

Not enough discharge flow Excessive discharge Head Not enough NPSHa Worn or damaged impeller Inadequate foot valve size. Air aspiration or air pocket in the suction line. Plugged impeller or piping... 

Two liquid control valve sizing rules of thumb are... 

Mote This table may serve as a rough guide only since actual flow capacities differ between manufacturer s products and individual valve sizes. (Source ISA Handbook of Control Valves" Page 17). 

When the control valve size is established by the designer, for example selecting a CV, it is assumed that this size valve is installed. If the control valve size is later increased it may be necessary to recheck the PR valve relief considerations. 

The quantity of material to be relieved should be determined at conditions corresponding to the PR valve set pressure plus overpressure, not at normal operating conditions. Frequently, there is an appreciable reduction in required PR valve capacity when this difference in conditions is considerable. The effect of friction pressure drop in the connecting line between the source of overpressure and the system being protected should also be considered in determining the capacity requirement. If the valve passes a liquid which flashes or the heat content causes vaporization of liquid, this should be considered in determining PR valve size. 

Boiler Steam Side Overpressure - All fired boilers are normally provided with PR valves sized to relieve the full steam rate in the event of closure of the normal outlet. 

If the control valve size is critical to the overpressure protection of the downstream equipment, and must not be increased, then this is clearly noted in all relevant documentation (specification sheets, flow diagram, operating manual, etc.,) and a warning notice plate is welded to the valve body. In such cases, an actual check of the valve installed or purchased should be made during the startup review. 

Sizing for Mixed-Phase Vapor and Liquid Service - PR valve sizing for a mixture of hquid and vapor at inlet conditions may be calculated using the sum of the orifice areas required for each phase individually, in the same way as described above for flashing liquid service. 

The "critical condition" in the specification sheet is entered as the contingency which determines the valve size, i.e., fire or operating failure. 

For example, increasing a valve size or installing a larger pump could result in high pressure in a vessel, thus increasing the risk of a release. Sanders (1993) presents a number of examples of changes affecting the safety of a plant. 

Water injection directly into the manifold eliminating a valve and reducing the valve size for drainage 3.3E — —... 

A vessel may be subject to more than one condition under different failure scenarios. For example, a low pressure separator may be subject to blocked discharge, gas blowby from the high pressure separator, and fire. Only one of these failures is assumed to happen at any time. The relief valve size needs to be calculated for each pertinent relieving rate... 

A vessel can only be overpressured if the upstream vessel has a higher pressure than the vessel in question. A compressor scrubber with a MAWP of 285 that gets flow from a 285 MAWP separator does not need to have a relief valve sized for blocked discharge. The upstream relief valve will keep the upstream separator pressure from going higher than 285, so there is no way it can oveipressure the downstream scrubber. The scrubber PSV only needs to be sized for fire. 

In summary, the back-pressure for relief valves should be limited to the following values unless the valve is compensated. We do not recommend using a relief valve with higher back-pressure than shown below without consulting a person knowledgeable in relief valve sizing and relief system design. 

Used to present mechanical-type details to piping and mechanical vessel designers, electrical engineers, instrument engineers, and other engineers not directly in need of process details. This sheet contains pipe sizes, all valves (sizes and types), temperature points, and special details needed to insure a common working basis for all persons on a project. In some engineering systems. 

Rather than assuming a pressure drop across the control as 25%, 33%, or 40% of the other friction losses in the system, a logical approach [9] is summarized here. The control valve pressure drop has nothing to do with the valve size, but is determined by the pressure balance (See Equation 2-59 [9]). 

G = specific graslty of gas (air = 1.0) or specific gravity of liquid (water = 1.0) at actual discharge temperature. A specific grasdty at any lower temperature will obtain a safe valve size. 

For relief valve sizing, refer to manufacturer s tables at 2.0 oz/in. pressure setting and 3.5 oz/in. maximum allowable tank pressure. 

Wong, W. Y, Safer Relief Valve Sizing," Chem. Engr., May 1989, p. 137. 

US) and 12 (English metric) are not exactly equivalent but are sufficiently close for practical valve sizing purposes. 

Boiler or water-heating (kW) Cistern size (1) Ball-valve size (mm) Cold-feed size (mm) Open-vent size (mm) Overflow size (mm)... 

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