Recycle Valve

[jaxdg]

I'm adding a recycle valve to keep a pump from shutting down on low flow. The following valve was recommended for the conditions listed below also:

3" 300# RF FISHER 667-ET, WCB BODY, 3-7/16" FULL PORT CAV III 1-STAGE TRIM, SST PLUG & SEAT W/ STELLITED SEAT AREA, SIZE 45 FAIL CLOSE ACT., 13-30# BENCH SET. PROCESS WILL FLOW DOWN OVER SEAT

P1 = 254 psig
P2 = 8.5 psig
T = 170*F
Q = 1000 gpm (Produced Water)

I was planning on using small bore pipe (3") to take some of the pressure drop. What is the maximum velocity you want going through 3" Sch 40 316 SS pipe?

 

[Cockroach]

You certainly want to feed the pump, I would be leaning towards finding the minimum volumetric flow required by the pump and then converting that number to a flow through a three (3) inch schedule 40 pipe. Probably adding 10% for theoretical uncertainties and losses wouldn't be a bad thing, you would like to err on the side of conservatism.

On the other hand, you can determine the state of choke flow on the three (3) inch schedule 40 pipe and reduce that to a level acceptable by the pump.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[BigInch]

That's a big recycle flow. I'd figure about 10-20% of pump capacity as minimum flow, if its centrifugal axial type. Higher % if not. Depends on if you will be running a long time on min flow? If your're at 10-20% flow, you need to look at heat rates if you're not going back to a large tank to get some mixing & cooling action. Fortunately you don't have much pressure drop. I'm not sure you need the anti-cav trim unless you get real hot. The valve seems a size small for that flowrate. I recently did one similar to this and used a 6" recycle line with a 4" valve, but I did have 2160 psi pressure drops with flashing gasoline, so maybe its not far off. If its 10% - 20% pump capacity discharge to suction, do not plan on operating for more than 15-20 minutes at a time.



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[TD2K]

Unless you have a LOT of pipe, you won't take much pressure drop over the SS pipe.

IF the water is clean, I might push 20 ft/sec but why? The control valve is going to have to take the majority of this drop and this is going to be a severe service valve no matter what.

If these are produced water pumps, I would recommend starting with the minimum flow from the manufacturer, I suspect they could be in the range of 30% to 40% of BEP to avoid suction recirculation problems. You can always decide to set the minimum flow lower if you like but start with the manufacturer's recommendations, minimum flows have increased significantly over when I first started in this business.

 

[BigInch]

TD2K is totally right. The problem is the "clean" part. I'm not worried about the pipe velocity either. Hi sand can take a valve out every 30-60 days.

The 30-40% min pump flows will not heat nearly as much as lower flows would and you'll get much longer op time, but still won't help sand through the valve.

I would say any recycle flows should not be "normal operation", in the sense that recycling should be used for starting and shutting down the pump... ONLY.

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