Piping Code Overpressure Allowance Beyond design Pressure PD
ASME B31.1 PD+15% if less than 10% of any 24 hrs.
PD+20% if less than 1% of any 24 hrs.
ASME B31.3 PD+33% if less than 10 hrs. once and less than 100 hrs/yr.
PD+20% if less than 50 hrs once and less than 500 hrs/yr.
ASME B31.4 PD+10%
ASME III PD+0% during normal operation PD+10% during upset conditions.
PD+50% during emergency conditions.
PD+100% during faulted conditions, with required shutdown.
These overpressure excursions are only permitted under certain conditions. For process piping, as an example, the conditions under which a pressure excursion is permitted are:
(a) no cast iron fittings in the system, (b) the stress should not exceed yield, (c) the longitudinal stress should not exceed 1.33S, where S is the code stress allowable, and (d) the pressure should not exceed the test pressure.
In addition, one must be careful when allowing overpressure to occur in pipe components such as valves or specialty fittings. In these cases, it is important to consult the manufacturer’s specifications to verify that pressure excursions such as those listed in Table 4-12 are permitted.
In the case of pressure vessels, if the vessel design pressure PD is also the relief valve set point, then:
PD−10%=practical maximum operating pressure.
PD−2%=the relief valve may start to open, the valve simmers.
PD+10%=as the relief valve discharges, the vessel pressure can accumulate another 10% above the valve opening set point. An ASME Section I boiler would only be permitted a 6% overpressure accumulation.
PD+21%=in case of fire, as the relief valve discharges, the vessel pressure can accumulate 21% (or 10% over the previously permitted 10%, in other words, 110%×0%=21%).
PD−7.5%=as the pressure decreases below the set point PD, the valve will start to close but may not completely close upon reaching PD. The valve may reseat as low as 7.5%
below PD. This is the blowdown region. For a boiler, blowdown would occur between 2%
and 4% of PD.
4.9 OVER-PRESSURE PROTECTION
Over-pressure protection of a component or system is most often achieved by the use of a pressure relief device (a pressure relief valve or rupture disc). A pressure relief valve may be (a) a relief valve which lifts gradually and recloses as the overpressure dissipates, which is typical of liquid service, (b) a safety valve that pops open suddenly, remains fully open, and recloses when the overpressure subsides as is typical in gas service, and (c) a safety-relief valve which operates either as a safety or relief valve. The approach to overpressure protection of piping systems is different than for pressure vessels. For piping, the system can be designed to “contain or relieve” the overpressure. In other words, if the pipe, fittings and components are sufficiently thick to contain all credible
pressure transients, the system does not need a safety or relief device. Control devices, such as pressure or temperature interlocks would assure that the design pressure of the piping system would not be exceeded. Because one has to postulate failure of control devices, these would have to be redundant in certain applications. For example, if the failure of a pressure regulator could over-pressurize a piping system, an alternative to a pressure relief valve would be a second regulator, in series, or an automatic shut-off device, in series. In the case of a centrifugal pump, we would have to assume that a downstream block valve fails closed or is closed by mistake while the pump is running.
We would then size the system between the pump and the isolation valve for the full dead-head pressure of the pump.
In the case of a pressure vessel, a relief device is required, even if the vessel is sufficiently thick. This requirement causes serious difficulties for pressure vessels containing fluids that corrode or stick to the relief valve or rupture disk, causing it to malfunction. To address this concern, ASME published Section VIII Code Case 2211, which addresses the question: under what conditions may a pressure vessel be provided with overpressure protection by system design in lieu of a pressure relief device as required by UG-125(a)? In the original issue of Code Case 2211, there were five conditions under which this “overpressure protection by system design” was permitted:
(1) The fluid may not be water, air or steam, (2) applying the code case is the owner’s decision, not the contractor, (3) system design must consider all credible overpressure scenarios, (4) the analysis must be made available to the jurisdiction, and (5) the vessel Data Report must refer to the Code Case. Note that in States with pressure vessel laws, the use of this Code Case must typically be authorized by the jurisdiction.
It takes a solid knowledge of a system and its operation to foresee the possible upsets that can occur and the resulting overpressure conditions. Sometimes, a system that has operated well for many years will be placed in an unprecedented overpressure condition.
The explosion in a dye plant, in Patterson, New Jersey illustrates this point [CSHI]. On April 8, 1998, a mixing process was underway at the dye plant. The mixing process takes place in a vessel, which is cooled by water or heated by steam circulating into an outer jacket. Figure 4-12 shows the entry of steam (ST) or cooling water (CW), and the exit of condensate (CO) or cooling water (CW). The vessel is protected by two rupture disks mounted in series (RD), and set to relieve at 10 psig [CSHI].
The mixture is introduced into the vessel and heated above 100°F by circulating steam through the jacket. As the reaction progresses, the temperature increases but is kept from reaching 380°F by switching the jacket flow from steam to cooling water. Temperature control is important since beyond 380°F an explosive exothermic reaction would take place.