re: 10" - 16" Manual Gate Valves "Double Seat" Trim Characte

[TurbineBlade]

Hi,

I would like find info about manual gate valves for isolation purposes. The gate valve spec. is Class 300 lbs SS body BW Sch40S Man Operated. The design conditions are T=200F and P=180 psig, in water. The Cv required is 3100. I came across with a similar spec that uses a trim characteristic of "Double Seat". My knowledge of valves is limited. Can someone help to understand what is "Double seat"?

In addition, can this trim characteristic achieve the Cv in a manual gate valve?

Thank you for your help in advance.

Patrick

 

[Ashereng]

Double seat sort of implies a slab gate or through conduit type gate valve to me. Typcially, when I purchase a slab gate, it is full port - so the Cv is that of an equivalent length of pipe (negligible).

Or, am I way off course?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[JimCasey]

Almost all gate valves are double seat. The gate drops in between the seats and is wedged into place by various mechanisms. Solid wedge, flex wedge, split wedge, and double disc parallel seat (with an internal wedge).

Knife-gate valves tend to be single-seat but certainly not used in Nuclear applications-usually slurry such as pulp.

here is a link so you can take a look at the construction of a gate valve:

http://www.flowserve.com/vgnfiles/Files/Literature/Products/HeritageBrands/Edward/Edward_Sect.C.pdf

Frequently Cv for gate valves is not even published because they are NOT for throttling. However, it would be highly unlikely for a 10" Gate valve to have a Cv that is not greater than 3100. As Asher said:essentially straight pipe.

Gate valves have metal-to-metal seats and it takes a huge specific loading to get them to shut off tightly. Basically it is necessary to re-coin the seat each time. Hence the wedge. Other handy associated devices for gate valves are the hammer-blow handwheel, and the large maintenance ape with the long cheater bar.

Review MSS SP 61 for seat leakage and testing procedures.

 

[BigInch]

My Cv's for gates

/* VALVE SIZE AND Cv=GPM/PSI^0.5 /* ENTER Cv ON DEFINE VALVE LINES
/* 1" = 48
/* 1.5" = 108
/* 2" = 192
/* 3" = 432
/* 4" = 768
/* 6" = 1728
/* 8" = 3072
/* 10" = 4800
/* 12" = 6912
/* 14" = 9408
/* 16" = 12288
/* 18" = 15552
/* 20" = 19200
/* 24" = 27648
/* 30" = 43200
/* 36" = 62208

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[TurbineBlade]

thanks for the responses.

 

[JimCasey]

Hey, Big Inch:
You table reminds me of the scaling rule of Cv vs valve size for a geometrically similar family of valves:

Cv=KD^2


D^2 is the valve size, squared.
K is a Constant, in this case: 48
so for your 1" valve Cv=48, for the 10" valve: 4800. Pretty close!

For globe valves, the K is usually pretty close to 14.

 

[BigInch]

Right. I just pasted those from some default input files I use. If I get specific mfgr data, (sometimes) I change them, but its never made a difference in a pipeline analysis so far, so generally I stick with those. I made the list based on some manufacturer's data from somewhere, maybe Daniels, but so long ago I don't remember now. The 1 inch could have been scaled down. Do the rest fit the curve exactly too? I've got files with ball and check fies ... defaults too. And default Cv curves for linear, EQPC and.. just enter the size and you get the Cv for any %Open.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[JimCasey]

If you try to match REAL data there is always the odd nonconformer that deviates some. There are singular effects like B16.34 causes some of the valves to have steeper approach transitions than others. But the KD^2 is a simple way to ball-park a Cv