what are the suitable gate valve for hot tapping?

[ahmedbu50]

hello everyone,


my company has selected a (flexible wedge gate valve, API 600) for the hot tapping. but the contractor who is in charge for construction said it should be (thru-conduit gate valve, API 6D.)now the client wants a justification form both side for why we have selected this type of valves. size of valve is 36", and the service is crude oil.

my boss give this job to me. i have done so many research about these two type of gate valve, but i did nor reach to a good justification for using them for hot tapping.

i will be glad if anyone could help me


thank you guys

 

[Danlap]

CMIIW, by hot tapping meaning that you want to perform some hot work on the downstream side while it is still on service.
May want to take below criteria for this kind of consideritaon:
1. Process condition (T, P, etc.)
2. Position of the valve (is it vertical or horizontal), some company preferred to installed it horizontally to prevent it from "locked" open or closed
3. Clean process crude oil or not.

By design, don't get me wrong, both valves (Through conduit and flexible wedge gate valve) perform good tight sealing in order for someone can work safely on the downstream side.
Through Conduit Gate Valve use the force from the upstream line pressure to make it more tight sealing (shall Differential Pressure taken into consideration). However, if the process is not so clean, once the valve retracted open, it may carry dirt which on most cases will scratch the seat (which eventually resulting internal leakage shall close position next time). And also if compared with wedge gate valve, the gate is a bit heavier, in that case shall positioned horizontally for some low end manufacturer, the stem would bent (due to load). And you know the rest.
Whether you choose Through Conduit or Wedge, should take into account the design and material of the seat, gate and stem.
I would suggest Flexible Wedge or Double Expanding Gate Valve, hence you are the Valve specialist who know the process and design condition.

 

[LittleInch]

I assumed you mean that this is the valve through which the hot tapping takes place, i.e. the shaft and the cutter is placed through this valve and then starts grinding its way through the parent pipe. Sometimes this valve is vertical, more often it is horizontal as the hot tap machines generally prefer vertical operation and it reduces the size of the excavation. The issue probably is one of risk that the metal particles and other debris from the cutting operation can lodge in the flexible gate valve preventing tight shut off much more than the thru conduit design which essentially creates a smooth bore and nowhere for any debris to gather. If this valve does not seal once the hot tap is complete, the hot tap machine cannot be removed from the hot tap without the main pipeline being shut down - normally the entire reason you are doing a hot tap is that you don't want to or can't shut off this main line.

If my assumption is wrong, then please advise, but the experience of the hot tap contractor should be listened to - they will have done many hot taps and had some bad experiences, so learn from them.

At the end of the day this is down to risk and cost difference between the two valves, but bear in mind that if you choose a different (cheaper) valve from that recommended by your hot tap company and it doesn't seal, any cost difference is likely to to be wiped out many times by the claim from the hot tapping company.... Your boss will promptly forget that he agreed to this and blame it all on you (!).

Because of their location (often below ground and very close to the pipeline, these valves are often just manual valves not normally operated with the process valves that open and close on regular basis ocated further downstream.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[pennpiper]

As LittleInch said "Ask the Hot-Tap Contractor, they have the experience"



prognosis: Lead or Lag

 

[uaepiping]

When hot tapping has to be done, a fitting is welded on the existing pipe (after checking a number of parameters including thickness at the point of hot tapping). These fittings have a flange at the top. On this flange a valve is installed which would then have Hot tap machine on top (or side depending upon the orientation of the hot tapping and new branching line). Let's assume that you are talking about this valve. Then,
As the hot tapping machine cutter has to pass through this valve before it reaches the EXISTING pipe, the passage for cutter has to be clear. Hence what contractor told you is right. It has to be through conduit valve (with nothing in the way of cutter).
This type of mistake happens, when MTO is prepared by an inexperienced designer (who would not write in remarks column or somewhere that this particular valve is for hot tapping), then person preparing the material requisition would simply refer MTO; take valve datasheet and place the order. If the valve happens to be full bore ball valve, or through conduit in the piping material class, day is saved else this situation!