Direct Burial of Gatve valve, is it possible at 8m deep - how?

[mddsnjc]

Hi all, I am looking for some advice on the direct deep burial of gate valves.

I have been asked to explore the option of placing a gate valve on an FST drain-down pipeline to allow controlled drainage of the tank during maintenance.

The only difficult bit being it is to be buried approx 8m below finished ground level. I have never dealt with buried gate valves and was looking for any advice or tips on a starting point. The pipeline will be MDPE with stub flange assemblies for jointing to the valve. An extension spindle to ground level will be used to operate the valve.

I am having trouble picturing the arrangement required when such deep burial is proposed if infact it is even possible?

Any info would be greatly appreciated, thanks for your time

Nick

 

[cvg]

if you want access to the valve for maintenance, then put it in a manhole, otherwise they are made to be buried. use a valve two piece valve box as shown

 

[mddsnjc]

Thanks for your reply. The valve does not require maintenence and so a manhole is maybe not cost effective. Do the valve boxes come in lengths of 8m? From what i can find they only seem to be short lengths say 2m max?

Thanks in advance for any further help

Nick

 

[cvg]

your supplier can provide the proper length using 1 or more extension pieces.

http://www.binghamandtaylor.com/pdf/castironcatalog11/castiron11p8.pdf

 

[BigInch]

Beware of torsional deflection of a stem length that long.
IMO you should really put it in a manhole. You say that you don't need maintenance, but what about when you do. The ultimate eventuality is that you will need maintenance. Pay now, or pay more later.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[Ron]

You will need maintenance on the valve at some point...after all, its purpose if for maintenance of the tank. It if is used infrequently, the seals will likely get damaged when it does get used. Such a long stem will likely deteriorate with time and as BigInch noted, torsion will be a problem...possibly resulting in stem failure. cvg and BigInch are correct...make it accessible.

 

[mddsnjc]

Thanks guys, my original thought was to make it fully accessible within a chamber as like you have said it will require maintanence/renewing at some point. I was tasked with a cost/risk comparison which lead to the evaluation of direct deep burial - something ive never designed for but it seems it is not the viable option when at this depth considering all the things that could go wrong.

Thanks again,
Nick

 

[cvg]

I don't see the manhole being a significant cost. you have to excavate and shore to install the pipe and valve anyway. Adding the manhole is only a small additional cost.

unless you add an electric valve operator, you will have a long extension stem anyway, but I think the torsion/stem failure issue is a bit overstated (unless this is a very large valve, but then you would have a gearbox).

 

[mddsnjc]

The cost of one does not seem to be a significant cost but I have 6 tanks and therefore 6 valves. 6 manholes could cost in the region of £30000 (approx) which on a project that is already over budget is quite a significant chunk. I think it would be possible to group the valves in pairs and get away with just three manholes with a greater diameter say DN1800. The spindle would just be hand operated as it is expected that the tanks will need draining only once per year and automation seems excessive. If in a manhole any issue with the spindle can be fixed. Direct burial would be the cost of the valve boxes and then an excavation cost should any issue ever arise. Its a cost/risk balance - how often does a DN200 gate valve go wrong? would all 6 fail over the expected life?

 

[BigInch]

cvg, So an electric operator can go 8m underground with no access? I don't get it.
It is not entirely unknown to shear valve stems (Murphy would add, especially when they are in unaccessable locations) and when they get a bit stuck and they have electric operators.

I don't think it's so much of an issue of "how often a valve fails". They certainly can fail ONE time, and BTW it's something you would be EXTREMELY unhappy about if they were buried 8 M deep. Even ONCE is once too often!

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[BigInch]

And I can add, when one fails, it will be at the exact same time when you absolutely MUST drain the tank. I can guarantee to you now that you will not have time to dig that 8 meter hole!

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[BenJohnson]

Ahhh, the old project over budget so look for a cheap solution problem. Gate valves do need to be exercised (open and close) on regular basis or the gate will freeze. This can be done with the two piece valve box suggested above. If budget money is so tight, then this is your only option.

It is impossible to do an accurate cost/risk analysis. Only God knows how long a gate valve will last without requiring major maintenance. If one does need to be pulled for maintenance or replacement, the cost will be far more than putting in 6 manholes.

I think that is a question for the owner. Your recommendation should be to use manholes .. not based on an engineering analysis, but rather that using a system which may cause major expense and real problems in the future. Better to do it right the first time. But if they want to save a little money, then go with the alternative.

 

[cvg]

biginch - of course not a buried electric operator, I meant electric operator in a manhole, to eliminate the need for an extension and also eliminate need for entering a confined space on a frequent basis to spin a handwheel...

if you are shearing off the valve stem/operator - it is because the valve is stuck / broken and you have an over zealous operator trying to force it.

 

[civilman72]

Unless I'm missing something, installing 25' (approx.) of manhole for each valve could very well be cost-prohibitive.

Assuming that turning the valve at this length is not an issue, what is wrong with a valve box with extensions?

 

[BigInch]

Yes. Does a bear s%#t in the woods. All operators fit the over zealous category when they are trying to open a stuck valve. They will not stop trying until the stem is sheared. It's the basic nature of being an operator.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[cvg]

I once saw a contractor place a 5' cheater bar on a wrench and then use the backhoe bucket to push on the cheater bar in order to tighten a bolt on a 30-inch steel flanged connection. The flange was warped and the gasket wouldn't compress, talk about over zealous.

 

[cnstengineer]

If you are putting the valves in manholes, I would recommend flanged pipe connections with pipe supports so that the valves could be more easily replaced in the future. If you go with direct bury, you can get away with mechanical pipe connections at the valve, valve box extensions, and valve key extensions. Attention to the plumbness of the valve boxes will need to be given during backfill. You should check with the manufacturer of the valve key extensions on the maximum length of extensions to be used. I've seen these available up to 20 feet.
Adding manholes at each valve with pipe supports, etc. will be very costly. The location of these valves should be considered for future maintenance. If they are out in a field area with not much around them, they would be more accessible than if they are in a paved area surrounded by other structures where expensive shoring means and restoration would be needed.

 

[BigInch]

cvg, Yes that is EXACTLY what I'm talking about. Jumping up and down, comealongs, chains, winches, 20 ft pipe cheaters. They will do ANYTHING... except walk away from it. It's like it becomes thier one purpose in life!

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[rconner]

I have to ask my more than likely stupid question. You mention something about valve use associated w/ "drain" of tank. Any chance to relocate valves, say away from very deep and immediately adjacent tank to another less worrisome location e.g on bank shore wall etc of whatever tank drains to (idea of flanges direct buried on polyethylene pipeline more than 26' deep gives me some Willies as well).

 

[mddsnjc]

It is a gravity pipeline from the final settlement tanks to a common pump station where the flow is then pumped back to the distribution chamber. It isnt possible to move the valves into a more accessible location because of this and also the fact we are working within a relatively confined space anyway. The area is heavily congested with pipework and so access to a deep buried valve may be an issue i.e. if it needed digging out and a revised pipe layout may be required. I am thinking of placing the valves within a manhole (2 in each) with an outfall to the original drain network. This will still require a long extension spindle but i guess if there was any problems with it at least someone could be winched down to change/fix it.