Spill back control valve for capacity control 1

[atedionyahoo]

Refer to API 618 capacity of reciprocating compressors could be controlled by unloading device, clearance pocket and spillback control valve.
If five steps unloading (i.e. 0, 25, 50, 75 and 100% of full flow) are used in combination of spillback control valve, it is kindly requested to inform us about following questions,
1. Which flows are reported as maximum, normal and minimum flow for control valve sizing?
2. Is it necessary to report full flow of compressor as a shutdown case for this valve and consider it to CV calculation?
3. If the response to item 2 is yes ,so which DP will be considered for valve in settle out (shut down)case?

 

[DLite30]

1. Difference between system design flow conditions and compressor flow capability will dictate what the flow requirements are for the valve. For instance, station operating flow condition is desired to be 200 MMSCFD and compressor unloading and speed control can only achieve a minimum of 250 MMSCFD at those conditions, then 50 MMSCFD at the stated suction and discharge pressures will be one point of consideration.

2. Depends on the installation. For stations with larger units, say more than 1500 hp each. there generally is a unit bypass valve used for compressor start up and shutdown. The station recycle or "spillback" valve is a seperate valve used only for flow control.

For stations with small units, say <1500 hp...you sometimes can combine the spill back and unit bypass valve into the same valve and generally will not be a globe type control valve, it should be a ball type control valve, and thus it needs to be sized for shutdown operation, for which there's no real cut and dry way of sizing. you'll have to look at your conditions to determine if this method is feasible...i.e. look at discharge temperatures with the spill back valve open, is there enough gas cooling, does the spill back valve need to tie in further upstream of the compressor.

 

[atedionyahoo]

thank you for your response to the questions .let me to describe the problem more detail.
we have 6 stages of compression from 2bara to 240 bara.
as you know,any stage have its own air cooler and the recycle btranch is located down stream of air cooler and return the compressed gas before K.O drum of compressor.
SO i think that there is not any problem about the cooling of returned gas,is there?
when inlet flow of compressor is equal to 0.25,0.5,0.75 of compresor capacity so 3,2,1 of suction valves will be ulodoad in scenario a ,b ,c(i.e 0,25,75%) respectively.but when inlet folw is between this pecentages, so portion of compensation will be done by unloaders and spiil back control valve will compensate remained portion.
for example when the inlet flow of compressor is 30% of compressor capacity,the capacity of compreddor is decreased to 50% by unloading of two suction valve.but the capacity of compressor is still more than existing flow.now remaind compensation (i.e 20% defrence betwean existing flow and capacity of compressor)shall be done as recycling of compressed gas by spill back valve.now i want to know since any compensation between each gap shall be done by spillback valve is it true to size the control valve for 0-25% of compressor capacity?on the other hand how can i ensure that this valve is suitable for shut down case ?

 

[DLite30]

Don't forget that unit rpm may also be another capacity/load control method available to you.

As for the scenario you described above, sizing the "spillback" valve, as you call it, for 25% of the flow rate of the unit may work. I size the valve such that the valve can flow that 25% when it is 80% open.

You will need to run all the different expected operating conditions of the unit to see what it's operating flexibility will be, considering flowrate, power, rod load, rod load reversal %/degrees, etc.

At that low of suction pressure, you'll probably end up setting the valve to maintain a certain suction pressure rather than a specific flowrate.

 

[DLite30]

Another thing as well, is that you'll also need to consider the temperature of the gas that you're spilling back to the suction. When it mixes with you normal suction stream, it may raise the suction temperature at the unit enough that you'll have unacceptably elevated discharge temperature.

 

[dcasto]

Dlite, most spill backs are downstream of the after cooler, so lower temps create liquids in some gases. Another gotcha.

 

[atedionyahoo]

Dear DLITE30
If I Size control valve for 25% of flow which is maximum flow passing controlvalve during operation,how can I ensure that the valve con serve suitable for 100% of compressor capacity during shutdown case?
on the other hand, we need to achive to settle out condition after compressor shutdown and blow down compressor in few secconds.so whcih DP shall be considered or expecetd for control valve in this case?

 

[DLite30]

To size the bypass valve, first you'll need to determine what the compressor flow capacity is with a small differential say 50 psid or so. Then use this flow and d/p to determine what size valve you may need. (FYI, plug valves work well as a bypass valve). You should play with the valve size, flow, d/p, and unit horsepwer to get the best valve fit. It's desirable to have a little bit of parasitic gas load when starting the unit.

Chances are that your bypass valve will need to be larger than your spillback valve, in which case the valve should be in parallel.

You may be able to use a single larger control valve to serve both functions, such that the spill back control range of the valve is more at the closed end of the valve say 0 to 30% open versus 0 to 80% open, however you may have issues tuning the controls for steady state or transient operation.

 

[bcs5274]

depending on temps recycle valve with wet gas and a large differential may freeze off also, another gotcha as stated above