cmcbain
Nuclear
- Dec 8, 2009
- 51
I recently inherited engineering responsibility for a poorly performing test assembly. The performance issues all result from a 4", 600#, Metal Seat, Floating Ball Valve that is intended to serve as a hydrostatic test boundary.
For background: I've attached a line diagram of my test assembly, all composed of 4" stainless steel piping. The purpose of the test assembly is to allow for testing of a relief valve (shown in the bottom right corner of the line diagram). The relief valve is overpressure protection on a steam system, so the test assembly is really just there to contain the steam.
During installation, a hydrostatic test is required to verify that there is no leakage at any of the flanged joints in the test assembly. The ball valve serves as the hydrostaic test boundary.
The problem: In my line of work, every component in every system has to be approved for use. The currently approved ball valve is, frankly, junk. A new valve will support anywhere from 1-10 hydrostatic tests before its leak rate becomes so excessive that it's impossible to achieve the required test pressure. Based on my research into how we ended up with this particular valve, it appears that a predecessor essentially purchased a valve recommended by a valve manufacturer with very little technical justification to back up the recommendation.
I'm trying to find out if I'm experiencing is just a fact of life with ball valves, or if I could find something better.
The Current Valve: Has the following characteristics.
4" IPS, 600#, Floating Ball
Body: 300 Series Stainless (2 Piece Body)
Ball: Ferralium 255 (not sure if its just a deposited coating, or if the entire ball is Ferralium)
Seats: Ferralium 255
The valve is made by a large and well known manufacturer, but it's considered a legacy product line, so I'm not getting very much engineering support.
The System: My working fluid is water, with a couple quirks. For reasons I can't control, there ends up being a small amount of high salinity water in the piping before the hydrostatic test. Even after filling the system with Deionized water for the test, the end product is probably water that has a higher than normal chloride content. There could also be any number of corrosion products or impurities in that small amount of water, meaning abrasives could end up deposited on valve sealing surfaces.
The hydrostatic test is performed at room temperature, but the valve will obviously become very hot during the use of the test assembly, as a result of the steam from the relief valve. I'd say up to 600 F.
In order to accomplish the hydrostatic test, I don't need a bubble tight valve. I do need a valve that minimizes leakage, and that can "seat" at a very low differential pressure/flow rate. My hydrostatic test pump can deliver < 300 cc/min, so I can't generate much differential pressure if there's a lot of leak-by to start out with.
I guess after providing all this background info, my questions are these:
1) Am I right in thinking a floating seat valve was never a good choice for this application? Seems like my puny test pump may not have the oomph to move a floating valve into place against a downstream seat.
2) Is it feasible to maintain a low leak rate through aobut 100 open-shut cycles of a metal seated valve? That's the number I'm shooting for, which would increase the number of test I can do to a maximum of about 25.
3) For those in the know, how do you feel that Ferralium 255 stacks up against the alternatives in terms of corrosion and abrasion resistance? The alternatives I see on most valve manufacturers brochures are various hardening procedures (Boronizing, Nobelizing), Inconel Alloys, Tungsten Carbide Coatings, and Full Ceramic valves/seats.
McBain
For background: I've attached a line diagram of my test assembly, all composed of 4" stainless steel piping. The purpose of the test assembly is to allow for testing of a relief valve (shown in the bottom right corner of the line diagram). The relief valve is overpressure protection on a steam system, so the test assembly is really just there to contain the steam.
During installation, a hydrostatic test is required to verify that there is no leakage at any of the flanged joints in the test assembly. The ball valve serves as the hydrostaic test boundary.
The problem: In my line of work, every component in every system has to be approved for use. The currently approved ball valve is, frankly, junk. A new valve will support anywhere from 1-10 hydrostatic tests before its leak rate becomes so excessive that it's impossible to achieve the required test pressure. Based on my research into how we ended up with this particular valve, it appears that a predecessor essentially purchased a valve recommended by a valve manufacturer with very little technical justification to back up the recommendation.
I'm trying to find out if I'm experiencing is just a fact of life with ball valves, or if I could find something better.
The Current Valve: Has the following characteristics.
4" IPS, 600#, Floating Ball
Body: 300 Series Stainless (2 Piece Body)
Ball: Ferralium 255 (not sure if its just a deposited coating, or if the entire ball is Ferralium)
Seats: Ferralium 255
The valve is made by a large and well known manufacturer, but it's considered a legacy product line, so I'm not getting very much engineering support.
The System: My working fluid is water, with a couple quirks. For reasons I can't control, there ends up being a small amount of high salinity water in the piping before the hydrostatic test. Even after filling the system with Deionized water for the test, the end product is probably water that has a higher than normal chloride content. There could also be any number of corrosion products or impurities in that small amount of water, meaning abrasives could end up deposited on valve sealing surfaces.
The hydrostatic test is performed at room temperature, but the valve will obviously become very hot during the use of the test assembly, as a result of the steam from the relief valve. I'd say up to 600 F.
In order to accomplish the hydrostatic test, I don't need a bubble tight valve. I do need a valve that minimizes leakage, and that can "seat" at a very low differential pressure/flow rate. My hydrostatic test pump can deliver < 300 cc/min, so I can't generate much differential pressure if there's a lot of leak-by to start out with.
I guess after providing all this background info, my questions are these:
1) Am I right in thinking a floating seat valve was never a good choice for this application? Seems like my puny test pump may not have the oomph to move a floating valve into place against a downstream seat.
2) Is it feasible to maintain a low leak rate through aobut 100 open-shut cycles of a metal seated valve? That's the number I'm shooting for, which would increase the number of test I can do to a maximum of about 25.
3) For those in the know, how do you feel that Ferralium 255 stacks up against the alternatives in terms of corrosion and abrasion resistance? The alternatives I see on most valve manufacturers brochures are various hardening procedures (Boronizing, Nobelizing), Inconel Alloys, Tungsten Carbide Coatings, and Full Ceramic valves/seats.
McBain
