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sewerratt (Civil/Environmental) (OP)
12 Sep 05 15:19
I am looking for the proper type of valve to install to bleed pressure off water mains when they have to be taken down for maintenance. The specific instance is a 72" main with a 6" tee and valve piped to open air to release water to drop pressure to zero. The problem I see is with a large volume of water (big pipe at 100psi) moving past a valve as it's opened, it's likely to have some damage to the gate. I see that RW gate valves specifically say 'throttling' is not an intended use. Any suggestions on a readily available municipal water works valve for this application?
Helpful Member!  stanier (Mechanical)
12 Sep 05 18:04
I suggestan eccentric plug valve. This has goood cavitation characteristics. Also it is good for wear as it is used on slurry applications. It will be 2-3 times more expensive than a gate valve. Dezurik make one but I am sure that there are other brands.

Alternatively the tortuous path CCI type boiler let down valve could be used but they are mega bucks.

Helpful Member!(2)  cvg (Civil/Environmental)
12 Sep 05 19:53
throttling is typically defined as using a valve in a partly open position to adjust the flow in a pipe.  Usually on the discharge side of a pump.  This is generally a long term setting for the valve.  If you are just opening the valve fully, occasionally to blow off pressure, it shouldn't be a big deal.  The valve can probably handle this.  Make sure you open the valve fully open.
sewerratt (Civil/Environmental) (OP)
13 Sep 05 8:14
I guess throttling is the wrong term. It's not long term use, right. But I've seen contractors burn the rubber on an RW gate while bleeding the pressure off the line. It wouldn't hold pressure afterward and the gate had to be replaced. Our maintenance people want the ability to slowly bleed off pressure without damaging the valve. So is an eccentric plug valve the proper application here?
cvg (Civil/Environmental)
13 Sep 05 11:25
If you have seen this damage before, then avoid that type of valve and a plug valve could be used.  A plug valve will work well for this and should have no difficulty shutting tight.  However, it seems to me that the valve must have been left in a partially open position for quite a while.  It should have been opened fully.
TBP (Mechanical)
14 Sep 05 10:43
I'm not familiar with the specific application, but I'm assuming the 6" valve is left in place after this procedure is done? If this is the case, I'd consider installing a pair of gate valves in series. The new one to be left in place, followed by the one used for this procedure. Both valves are shut to begin with, open the valve to be left in service first. The only flow you'll get is the little burp while the air/water fills the space between the valve bodies. Use the second valve to take the wear. After you're done, close the valve you've been using to throttle, then close the valve to be left in place. Dump the pressure between them, pull the throttled valve off, and use it for the next throttling job. I'd be that a pair of 6" gate valves is cheaper than one 6" plug valve, plus you can reuse one of the gate valves on the next throttling job.
JimCasey (Mechanical)
14 Sep 05 14:35
I am not understanding.  
the 72" water main is full of water which is only marginally compressible.  Isolate the water main, open your vent valve on the 6" Tee, and you'll lose one or 2 teaspoons of water as the water main vents to atmospheric pressure.  If you want to DRAIN the water main, that will take longer.  You'll have to open a vent valve and a drain valve, and provide someplace for the water to go. None of this has to be done with a throttling  valve, and the most throttling that will occur is isolating the 72" main slowly enoufh to avoid water hammer.  

Personally I hate gate valves for almost ANY application, because they almost invariably have metal-to-metal seating surfaces which are so likely to leak.  I'll take a ball valve or a butterfly valve or a plug valve anytime for isolation if the valves are rated for the process conditions.  On the 6" line, the Ball and Plug valves might be a it pricey, but the Butterfly will be economical.  
ap76 (Materials)
14 Sep 05 14:37
I don't see where you would need to throttle a 6" valve to drain a 72" main.  It is going to take a long time to bleed pressure off of a main that large with a valve that small.  The last application of a drain gate valve that I was involved in we used a 12" valve to drain a 48" main.  With respect to throttling, there is one AWWA C509 valve manufacturer that does not recommend against throttling, but being to to this forum, I don't know if I can name them.

One other thing, using a plug valve will restrict water flow and slow the bleed off process due to the the fact that the valve only gives you an opening about 80% of the diameter of the pipe where as the gate valve is a "fully open port"  Also most plug valves are used in sewer service and may or may not meet the requirements of NSF 61 so you may want to keep an eye out for that too.

Can the section of line in question be isolated on side from the supply side first to bleed most of the pressure off prior to opening the drain valve?

sewerratt (Civil/Environmental) (OP)
15 Sep 05 18:03
Theoretically, a teaspoon or so is the case. In practicality, a LOT of water will run out. A little more information. This is an existing line with some age on it. There is about 2000 ft between inline gate valves. the blowoff is at the low spot it the line, near where it crosses a creek before it crosses under a RR. It is 17 ft deep in mucky soil with ground water. I don't want to dig it up again. The line section can be isolated by the inlines, but it is a transmission main. There is still line pressure on it. When you bleed pressure off a line, the bleed valve is opened slowly, due to the practical nature of opening the valve plus the maintenance issues of sudden pressure changes on a line.

BTW, metal to metal are disc valves and they leak a little but they are OK for this application, aside from the leakage, which makes it bad as a valve. the gate valve in question is a resilient wedge valve.

From what I've seen here, a plug valve will be my choice on the next installation and when we repair the current RW valve. The additional cost is not an issue as this application is not common in our system. The repair will be a nightmare, but the new plug valve should prevent that from reoccurring.

Thanks to everyone for the advice and help. It has been most useful.
joetbear (Mechanical)
23 Oct 05 22:48
It seems that most of the responses are from people that have not worked the field much. Stick with a gate valve, at the higher pressures, you might need to put in bypass line, as for the blowdown, a strainer and PR valve can be attached easly. Bleed down the line with the PR valve, then remove it. You need a gate valve for the blow off, for the duty rating and scouring effect.
biglil (Civil/Environmental)
12 Dec 05 9:13
industry standard is to install blow-offs (APCO is a mfgr that is widely used in SoCal and they have design/sizing tools) at lowpoints for "draining" a water main during shutdown.

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