V. Prabhu, Worley India Pvt. Ltd., Navi Mumbai, India
Designing a pressure safety valve (PSV) for any project is a safety critical activity carried out by a process engineer. PSV calculations comprise several distinct steps, including credible scenario identification, required relief load calculation, estimation of effective orifice area and designation (using API Standard 520 Part 1), and determining the preliminary sizes of safety valve inlet and outlet pipework.
Normally, the focus is on scenario identification, calculation of relief load and an estimation of PSV size. Often, not enough attention is given to PSV inlet/outlet lines except limiting the pressure drops within American Petroleum Institute (API)/American Society of Mechanical Engineers (ASME) recommended limits.
The purpose of this article is to explain additional process considerations for pipework connected to PSVs, such as isolation valves, layout, orientation, forces and moments.
INLET PIPEWORK
PSV stability is the most important design criterion for inlet pipework. All the reasons behind PSV instability and the failure of inlet piping are not fully understood.
Applicability of the 3% rule for inlet pipework. Some owner companies allow up to 5% inlet losses on existing valves. However, the 3% rule is considered as a Recognized Good and Generally Accepted Engineering Practice (RGAGAEP) by the U.S. Occupational Safety and Health Administration (OSHA). Engineering analyses are required if pressure drop exceeds 3% of the PSV set pressure. These analyses are complex and require details of valve inertia, and opening and closing times of valves that are often unavailable.
With pilot operated relief valves (PORVs), when the pilot sensing point is at the valve inlet, pop-action and many modulating action pilot-operated PRVs have the same 3% inlet loss criteria as conventional and balanced bellows-type PSVs.
Higher inlet losses may be acceptable if the pilot sensing point is remote from the valve. The pressure loss from the vessel to the sensing point must still meet the 3% maximum criteria. The reduced pressure at the PORV inlet must be accounted for in valve sizing. Some modulating PORVs can tolerate > 3% inlet loss. The extent of the tolerance must be defined by the manufacturer and, again, the reduced pressure at the PORV inlet must be accounted for in valve sizing.
Precautions during PSV installation. The PSV should not be placed near flow disruptions to avoid setting up acoustical resonances that can cause the PSV disk to vibrate and, in some cases, lift slightly off the seat. This can result in seat damage and leakage.
Some relief valve manufacturers recommend that tee connections to inlet lines have a rounded entrance to minimize the formation of acoustical resonance. Poorly supported piping or nearby mechanical equipment may cause vibrations in relief valve inlet piping. These vibrations may cause leakage in the PSV seats or fatigue failure of the pipework.
High-frequency vibrations—regardless of amplitude—are detrimental to safety valve tightness. The effect can be minimized by providing greater pressure differential between the operating pressure and the PSV set pressure.
Install snubbers or other vibration reducing devices if pulsating flow is expected (i.e., discharge of reciprocating compressors and pumps). Inlet pipework requires heat tracing and insulation if process fluid is viscous/congealing or tends to cause corrosion on condensation.
Outlet pipework. The main criterion of outlet pipework sizing is that the total backpressure (superimposed plus built-up) that may exist or be developed in any credible relieving scenario should not reduce the relieving capacity of the PSV. The built-up backpressure caused by the flow through a PSV should not cause any instability in valve operation.
The backpressure should not exceed the mechanical limits of any piping or PSV component, as identified by the valve manufacturer. Bellows of large valves are often not rated for the full pressure rating. For instance, a standard pressure rating for bellows of an 8T10 valve from a common manufacturer is 2 barg (30 psig), and the high-pressure bellows option is rated for 4 barg (60 psig).
PSVs in liquid service can be evaluated based upon the required relief load rather than the valve-rated capacity when the liquid flow capacity is limited (e.g., in a positive displacement pump).
Although API Standard 520 Part 2 does not explicitly define the Mach number limitation for a PSV tailpipe, a good engineering practice is to restrict the Mach number of outlet pipework to < 0.7 for PSV-rated capacity.1
Inlet and outlet pipework lengths for pressure drop estimation. FIGS. 1 and 2 provide conservative pipe lengths of PSV inlet and outlet pipework in absence of piping isometric sketches.
Isolation valves. Where a PSV is not spared, an inlet isolation valve is recommended to allow easier and safer PSV isolation for removal for inspection and testing. No inlet isolation valve is needed on spared equipment if the customer is willing to switch to the other equipment if the PSV needs service. Note: Inlet isolation valve are not permitted for PSVs protecting ASME Section 1 vessels.
Double isolation may be required in high-pressure or hazardous service. Check customer standards for PSV isolation requirements.
When a spare PSV is provided and relief is directed to the flare system, both PSV inlet and outlet piping should have isolation valves. Isolation valves in service of the PSV inlet should be car seal open (CSO)- or locked open (LO)-type, whereas isolation valves in service of the spare PSV inlet should be car seal close (CSC)- or locked close (LC)-type. Isolation valves in both services as well as the spare PSV outlet should be CSO- or LO-types (FIG. 3).
It is preferred to provide suitable mechanical linkage between all PSV inlet and outlet isolation valves to improve reliability. Isolation valves, if used at the PSV inlet/outlet, should be a full port (bore)-type. Reduced port valves should not be used.
Only gate or ball valves are permitted as PSV isolation valves. Globe or butterfly valves should not be used as they are not full port due to the presence of internal elements.
Layout. In columns or pressure vessels, a PSV is located on the top. In a column, it is located on the overhead vapor line (FIG. 4). If a vessel has a demister pad, there is a possibility of blockage of the relief path due to its dislodgement. The demister pad may get clogged if the service is dirty or has the tendency to plug. In all such cases, the relief valve should be located in the vapor space below the demister pad.
The PSV inlet piping should be free-draining towards the piping or equipment on which it is installed. No pockets are permitted. If the PSV discharges to the atmosphere, the discharge must be taken to a safe location.
For vapor relief, a safe location is 3 m above the nearest platform or grade where a person may be located within a 15-m radius. A minimum 10-mm diameter weep hole must be provided in the horizontal portion of the outlet piping. This prevents the accumulation of liquid that might freeze and obstruct the relief path.
Liquid relief is directed to the grade. All of these requirements should be shown in the piping and instrumentation diagram (P&ID).
Orientation of PSV isolation valves. The gate valve, if used for isolation, should be installed with its stem in the horizontal position (preferred) or a maximum 45° downward angle from the horizontal plane—this is to avoid the gate from falling off and blocking the relief path (FIG. 5). This also requires a notation be added to the P&ID.
PSV laterals should enter flare headers from above. In the past, it was preferred to enter the flare headers at an oblique angle (30°–45°) to the header, but it has been found that the mechanical joint is difficult to fabricate correctly and so has an increased risk of failure than a forged tee: forged tees are now preferred.
Reaction forces and moments. If the sound power level (SPL) in the outlet piping is below 155 dB, no further analysis is required.
An acoustic induced vibration (AIV) study should be performed for any vapor PSVs that produce more than 155 dB to ensure the integrity of the piping. Mitigation measures include an increase in piping wall thickness, and replacing fillet welds (such as weldolets) with butt welds (forged tees and swages). In extreme cases, the line size is increased to avoid a sonic shock, or separating the flow into multiple valves may be needed. HP
LITERATURE CITED
VINAYAK PRABHU is a Deputy General Manager, Process, at Worley India Pvt. Ltd. He has more than 28 yr of work experience as a process engineer in engineering, procurement and construction (EPC) companies. Prabhu earned a B.Tech degree in chemical engineering from Nagpur University, India, and is a Chartered Engineer (CEng MIChemE) in the UK. The author can be reached at Vinayak.Prabhu@worley.com.