Relief Valve Discharge Pipework Pressure Drop 1

[BenjyMac]

I am currently designing a relief valve discharge pipework for a steam line. Please could someone tell me if the following is true:

The flowrate used for the discharge pipe sizing should be at least the Certified Discharge Capacity of the relief valve (I know some would use the un-derated figure).

I ask this question because the discharge pipework contains an expansion chamber to remove stresses from the pipework. When I carried out a blow-back check for the expansion chamber I realised that the vendor who had supplied the silencer had designed it based on a flow of 30,000 kg/hr which was a required relieving capacity giving a pressure drop of 0.4bar. The relief valve Certified Discharge was 47,755 kg/hr which meant a pressure drop 1 bar. The vendor argues that the discharge pipework would never see that flowrate because the required relieving rate (from a package boiler) would never exceed 30,000 kg/hr.

I think he is wrong and the pipework (and fittings) should be designed at 47,755 kg/hr. Am I right because at this flowrate we have a problem of blow-back. What are people's thoughts to replacing the expansion chamber for a bellows or flexi?

Many thanks in anticipation

Paul

 

[TD2K]

Couple of points, API 520 states that inlet and line loss calculations are to be done at the valve's rated capacity, not the required relieving capacity.

In addition, while it is true that over the long haul, you can't relieve more steam than the boiler is producing, that's not true in the short term. As you approach the set point, the valve will start to simmer and then pop open if it is a conventional relief valve. Given the difference between 30,000 kg/hr and the valve's capacity of 47,755 kg/hr, you might not get to 10% overpressure but I would be hard pressed to argue taking credit for less than the valve's calculated capacity at an inlet pressure = set pressure. That's still going to be more than 30,000 kg/hr.

As the valve starts to relieve its rated capacity at some overpressure, the inlet pressure is going to fall causing it likely (in this case given the capacity difference) to close (basically the question is: at what inlet pressure does the valve pass 30,000 kg/hr relative to its blowdown setting). Then, it will reopen and cycle. What the maximum flow rate would be in the tailpipe during this sort of relief would require a transient analyses.

Now, if the valve is a modulating relief valve, it will open only enough to maintain the inlet pressure at or slightly above set pressure (AGCO has told me this is typically within 2% to 3% of set pressure) and in this case, you might be okay. But, if future relief loads increase, while the PSV is large enough, you could run into a problem with the bellows and you still have the issue of what API 520 states.

 

[CHD01]

TD2K is 100% correct. The transient load on the discharge line at the relief valve capacity may last < 1 sec - but this load still needs to be considered in the design of the supporting steel.
The more you learn, the less you are certain of.