Assessing Pressure Relief Needs

Evaluating thermal expansion and overpressure protection.

By Chip Eskridge, P.E.

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As engineers work their way through a tedious process hazard analysis, an often-raised question is: "Does this line [i.e., pipe] need a pressure relief valve [PRV] for thermal expansion/overpressure protection?"

Faced with the voluminous requirements and convoluted arrangement of the American Society of Mechanical Engineers (ASME) codes, as well as unfamiliar or often misunderstood requirements of the governing jurisdiction (e.g., the state), most engineers act conservatively by installing one. However, overzealous use of PRVs can cut needlessly into capital budgets and, more importantly, the valves’ associated periodic testing requirements can become ongoing nuisances and costs.

Although ASME pressure vessels generally require PRVs, the ASME B31.3 Process Piping Code does not mandate PRV installations in a piping system in all situations. In addition, PRVs are not mandated by most state laws.

However, the expansion of a trapped liquid resulting from a temperature increase can translate into high pressures and sometimes equipment damage. This can be attributed to the relative incompressibility of the liquid and the relative rigidity of the pipe and is of concern only when a system is full of liquid, isolated and heated.

Although both the ASME Section VIII Boiler and Pressure Vessel Code and the ASME B31.3 Process Piping Code require the designer to consider all overpressurization scenarios, including thermal relief, no specific discussion or formulas are provided because those parts of the code are performance based.3 Moreover, the codes direct the user to American Petroleum Institute (API) Recommended Practice (RP) 520 and RP 521, which do not cover the subject in sufficient detail.

PRV requirements

The ASME Section VIII Boiler and Pressure Vessel Code leaves very little room for the omission of a PRV, other than ASME Code Case 2211. However, the ASME B31.3 Process Piping Code does not require piping systems to have a pressure-relieving device as long as the system is designed to withstand the highest developed pressure. For many piping systems, the highest developed pressure (maximum expected pressure) is determined easily by examining the pump shut-off head.

When determining whether or not a piping system requires a PRV, the engineer must compare the highest developed pressure to the weakest component in the system. If the weakest component can withstand the highest developed pressure, a PRV is not required. An often overlooked exception to this is the occasional variance clause in ASME B31.3, which allows a system to be overpressurized occasionally by 120 percent of design pressure, not to exceed 500 hours per year (hr/yr) (no more than 50 hours at a time), and 133 percent of design pressure, not to exceed 100 hr/yr (no more than 10 hours at a time), under certain conditions. As a general rule, when a system is designed to have flanges, the flange ratings often are the weakest component. When a system does not include flanges, a block valve or a process instrument often is the weakest component.

Determining the "weakest" component

For a 150-pound (lb)-class flanged system, an ASME SA-105 carbon-steel flange is rated for 285 pounds per square inch gauge (psig) up to 100°F, and derates linearly with temperature to 260 psig at 200°F. A properly torqued flange will not deform, but usually will leak or "blow a gasket" when overpressurized. This should occur at approximately 500 psig. Overpressurization resulting from thermal expansion can occur easily at this pressure.

On a nonflanged system, when piping is joined through either welding or threading, the weakest component often is a block valve or a process instrument. Usually, the pipe itself can withstand many thousand of pounds of pressure. The required pipe wall thickness can be calculated using Equation 1:

(1) t = PD/2(SE - PY) or P = 2SEt/D + 2Yt

where:
 P = maximum allowable internal pressure of the pipe (psig).

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