THIS MONTH'S PUZZLER
A safety expert from headquarters surveyed our refinery's pressure relief system and took issue with a number of practices, including use of a check valve for thermal relief, connecting blowdown lines to the discharge of a pressure safety valve, and having several relief devices share a common discharge (Figure 1). Operations dismiss the expert's comments as quibbling to justify the survey and maintain nothing is wrong with the current solutions to these pressure relief problems. Please help me convince them these really are problems.
THE PROBLEMS ARE REAL!
Let's address each specific concern, starting with the check valve used for thermal relief. Check valves are notorious for leaking. In this case, you have process pressure against the "flap" of the check valve if it is mounted to open in the direction of flow. Depending on the spring loading, the check valve is leaking water continually. If there is a heat exchanger leak, you could have an air permit violation. You need a pressure safety valve (PSV).
Connecting blowdown lines to the discharge of a PSV is borrowing trouble. Material will accumulate at the downstream side of the PSV. The relieving capacity of the PSV will be reduced, possibly to a point where it can no longer protect the equipment it was designed to protect. The inlet upward force from the relieving pressure has to overcome the downward force — the PSV spring force and the backpressure — to lift. If material accumulates in the header, you have increased the downward acting force to a point where you have compromised the mechanical integrity of the vessel being protected. A process hazard analysis (PHA) or layer of protection analysis (LOPA) should have captured this problem, requiring the routing of the blowdown streams away from the PSV discharge.
Having several relief devices share a common discharge is a similar problem. When relief valve (RV) discharge lines are common, the designer must account for the combined relieving flow from each RV and its impact on other valves. You have effectively increased each RV's amount of built-up backpressure, the pressure created by other flows in the network, which could impact its relieving capacities. Backpressure is built-up and super-imposed. Normally 10% is allowed for overpressure; RV flow is at set pressure plus overpressure. Fire case sizing allows 21%. For a conventional RV, the backpressure cannot exceed 10% of the set pressure of the RV or the flow capacity will be reduced. If, by combining the flows for each RV, the backpressure exceeds 10% in the vent the capacity of those RVs will be reduced and the mechanical integrity of vessels will be compromised. I would suggest revisiting the PHA/LOPA for these designs and I would also have the safety expert participate in the discussions.
Eric Roy, principal engineer
Westlake Chemical, Lake Charles, La.
CAREFULLY CONSIDER BACKPRESSURE
The operations personnel who do not accept the expert's comments as valid should thoroughly read API Recommended Practices — API RP 520 and 521 — and other relevant international guidelines. In general, referring to all the examples, it should be highlighted that the performance of a relief device significantly depends on the backpressure that occurs during the relief scenario. API RP 521 requires that backpressure for a conventional PSV be limited to 10% of the set pressure. A balanced PSV, such as a pilot-operated valve, can take a higher backpressure. Backpressures on each relief device in the refinery should be evaluated with peak relieving flow rate for a conservative and credible concurrent relieving scenario such as emergency shutdown at maximum operating, trip pressures or PSV set pressures. Higher flow rates at the discharge would also mean higher fluid velocities in the PSV tailpipes, which could lead to acoustic-induced vibrations and flow-induced vibrations. Sonic velocities in the PSV tail pipes can cause fatigue failure damage depending on the type of stresses. These calculations can be done with the aid of software. Let's consider the thermal relief case. A check valve may not be acceptable as thermal relief depending on the service (e.g., flashing liquid). A smaller size relief valve such as ¾ in. × 1 in. is often installed for thermal relief.
Nikhil Barbare, senior process engineer
Transfield Worley, Perth, Australia
ADHERE TO CODES
Let's start from upper left to right of Figure 1. Check valves are no substitute for relief devices or as inlet or outlet valves. API RP 520, Part II, 5th edition, Section 6.3.1, states: "Valves shall have the capability of being locked or car sealed open." Obviously, check valves don't qualify. Replace the check valve with a PSV set to the maximum allowable working pressure of the heat exchanger. In the second case, isolate the PSV discharge from the blowdowns with a tank (Figure 2).