4 replies to this topic

[Bill B]

I have a situation that has arisen several times:
1. RV well oversized and inlet dP and back pressure are higher than 3% and 10%.
2. At the scenario rates, the pressure drops are insignificant.
3. Importantly, at the maximum possible rate the dPs are acceptable.

Example 1 - compressor capacity - 300 units, RV sized for 1000 units.

Example 2 - flash tank - RV sized for 1000 units, governing case (flashed steam) = 100 units, rate of steam generation if all of condensate flashed (I realize this is non-sensical) = 300 units.

So, in case 1, only 300 units can physically be "pumped" to the RV. In case 2, even if all of the water were flashed, only 300 units per hour of steam could be generated.


Are there any provisions for discounting the 3% / 10% inlet and back pressure API rules?

Thanks.

[Dazzler]

Hi Bill

When we design relief valves, we determine the required flow (highest possible scenario). Then a standard valve size is selected which needs to be able to provide higher than the required flow. So for the selected valve at the same relief conditions we work out the rated flow for the valve. The inlet and outlet pressure drop ratios are calculated for this higher flow. This is important because when the valve opens and lets down the pressure from the protected system, the flow is the rated flow, not the required flow, until maybe later if a steady state relief state is reached and only the required scenario flow continues. RV performance and reliability is assisted by following the API pressure drop guidelines which require the pressure drop ratios to be determined from the rated flow.

You mention "maximum possible flow" - which I would have thought is the rated flow from the valve.

Dazzler

[fallah]

I have a situation that has arisen several times:
1. RV well oversized and inlet dP and back pressure are higher than 3% and 10%.
2. At the scenario rates, the pressure drops are insignificant.
3. Importantly, at the maximum possible rate the dPs are acceptable.

Example 1 - compressor capacity - 300 units, RV sized for 1000 units.

Example 2 - flash tank - RV sized for 1000 units, governing case (flashed steam) = 100 units, rate of steam generation if all of condensate flashed (I realize this is non-sensical) = 300 units.

So, in case 1, only 300 units can physically be "pumped" to the RV. In case 2, even if all of the water were flashed, only 300 units per hour of steam could be generated.


Are there any provisions for discounting the 3% / 10% inlet and back pressure API rules?

Thanks.

What is your main concern?

-"The valve being oversized" or "the pressure drop in inlet/outlet line is higher than 3% / 10% of the set pressure"!

Edited by fallah, 14 April 2010 - 01:21 PM.

[Bill B]

Thanks for the feedback. I am in the process of evaluating several hundred valves, and have run into this situation several times - where the RV is very much oversized and the inlet / back pressure is > 3% and 10% based on the rated capacity of the RVs. For example I have a scenario requiring a gas rate of 4100 pph be handled, and the RV has a rating of 35000 pph and the dP is high. The dP at 15000 is fine.



I hate to spend resources replacing valving and piping under these circumstances. Thanks

[fallah]

Thanks for the feedback. I am in the process of evaluating several hundred valves, and have run into this situation several times - where the RV is very much oversized and the inlet / back pressure is > 3% and 10% based on the rated capacity of the RVs. For example I have a scenario requiring a gas rate of 4100 pph be handled, and the RV has a rating of 35000 pph and the dP is high. The dP at 15000 is fine.



I hate to spend resources replacing valving and piping under these circumstances. Thanks

Based on a good engineering practice the minimum relief among all applicable scenarios for a PSV should be at least 25% of its rated capacity,otherwise that valve would subject to chattering.In those cases you can use two or more valves instead one with staggered set points.

For high pressure drop in inlet/outlet lines you can use larger size.