What is the Double Block and Bleed Procedure? | Podcast Episode #13

double block and bleed procedure
In this episode we are joined by Kyle Andrews, Product and Applications Trainer at Kimray, who explains the double block and bleed procedure to isolate a valve before performing maintenance.

Topics in Double Block and Bleed Procedure

  • When is the process needed?
  • Why is it recommended?
  • Why do some operators skip this step?
  • What can happen if this procedure is not followed?
Resources mentioned in this episode:

oil and gas podcastblock and bleed valvedouble block and bleed procedure


Curtis: Welcome to Stuff You Should Know about Oil and Gad Production. I’m your host Curtis Winkle. Today’s topic is a really important safety measure that we want to talk about. It’s this double block and bleed procedure. So, Kyle, why don’t you start us out—why would someone be working on a valve, first of all? What might they be doing?

Kyle: It could be that they’re repairing a valve, changing trim packing. There’s a multitude of things that they could be doing to the valve that would require them to bleed the pressure and make it a safe working environment.

Curtis: And basically anytime you’re going to do something to a valve, you need to practice this safety measure, right?

Kyle: Yeah. Anytime that you’re going to be opening not only a valve but anything that contains pressure, this is an important practice.

Curtis: Do people try to avoid it for some reason? Are they thinking like, I can take a shortcut here to just adjust this?

Kyle: Right, people are tempted to take shortcuts to speed it up. But it’s really dangerous to do that because there’s the potential that pressure could still be present. Pressure could be coming from another place that you’re not expecting.

Curtis: And this is a big reason we do this, so let’s just walk through this. I know I’m going to have to check this valve. Maybe it needs a repair kit. What are the steps I need to take before I actually open it up?

Kyle: Double block means you’re blocking both ends, upstream and downstream of the valve or the piece of equipment that you’re working on. So, you want to shut off the upstream pressure, and usually it’s a ball valve that does this. Now shut off the downstream by closing a ball valve or something to block downstream pressure. Now, you’re blocked on both sides of your valve.

Curtis: And how far away from the control valve might those ball valves be?

Kyle: Usually it’s right next to the ball valve. There’ll be a pipe nipple or some kind of flange mated up to the valve. But it’s usually within an arm’s length on both sides on both sides. Producers do that just to make it easy to get to. So it’s real simple to close both of those ball valves.

Now, you must deal with the pressure that is trapped in between those two ball valves. You must bleed the pressure, after you have double blocked it.

Curtis: How do you do that?

Kyle: On a lot of our control valves, there are two ports in the body, one on the upstream side and one on the downstream. A lot of producers will have needle valves, with a piece of tubing coming out. They can put a bucket under it to catch whatever comes out of the valve. This is a place to bleed off the pressure that’s trapped in the valve.

Curtis: When we talk about working on these valves and that you need to follow the double block and bleed procedure, is it specifically certain types of valves? I’m thinking like a dump valve? I don’t know if you’re going to do it there. 

Kyle: It could be a mechanical dump valve. It could be a high pressure control valve. If you’re ever going to work on something such as a liquid level controller or maybe a trunnion assembly that’s inside the vessel, you have to take these practices and utilize them on the entire vessel.

For example, on something like a separator if you’re going to be taking out a Gen II Liquid Level Controller, you can’t just unthread it and take it out because that whole vessel has pressure on it. So, you must block the upstream of the vessel and basically bleed the pressure off of it, which depending on the size of the vessel could take quite a bit of time.

Curtis: Would they have a needle valve on the vessel itself or is that come through the valve too?

Kyle: Usually if you’re draining and depressurizing an entire vessel you can do that through the back pressure valve, basically. You lessen the set point of that vessel by using the adjusting screw on the regulator to effectively put the set point at zero.

Usually there’s a ball valve or some kind of valve to release the remaining pressure on that vessel. Now, if you’re taking out anything that goes into to the vessel, like a liquid level controller trunnion assembly, you also have to drain the liquid level to below that connection point.

Because even though there may not be any pressure on that vessel, if you unthread a liquid level controller and the level of the fluid level inside the vessel is right there, you’re going to have a spill. 

Curtis: So you’re watching the sight glass to see, make sure it’s lower? 

Kyle: Before you depressurize the vessel, you want to make sure that you drain the liquid to an appropriate level. Because if you have no pressure on that vessel, you can’t get rid of the liquid because it’s not able to push that liquid out. So, you want to get the liquid level down to an appropriate level and then depressurize the vessel.

Curtis: We talked about this a little bit, but what could happen if they don’t practice this double block and bleed procedure? 

Kyle: So maybe somebody is trying to cut a corner and save some time, so they just shut off the upstream pressure. Well, you may think “no pressure is going through” right? “I’m getting no pressure from upstream so I’m good to work on this valve”. But there could be pressure downstream of that valve. Let’s say for example it’s a dump valve that you’re working on. So, you shut off upstream of it and downstream there’s just storage tanks so you just think, “Oh, they’re just atmospheric storage tanks”. 

Even on storage tanks there could be some pressure. There could be liquid in that part of the, the valve body, because of how far it could be to the tanks, it could have some liquid head pressure. If there wasn’t a check valve installed and you were to take that valve apart, it could push that liquid back out of the valve onto you.

Curtis: Just like a water tower, right?

Kyle: Absolutely. So, you want to make sure that you block both sides of the valve. A lot of the times, even with double block and bleed procedure, if you’ve blocked both sides of the valve and you bleed the pressure off and you start taking apart the valve, if you ever hear a hiss or if there’s signs that there’s still pressure, the ball valves could have bad seals so you could potentially be leaking pressure.

Curtis: So in that scenario, you want to put everything back together and then check the ball valves, right?

Kyle: Basically, anytime you hear a hiss, tighten everything back down. Don’t continue to take apart the valve because that means there’s still pressure. There is still something going on in there that it needs attention.

With some of our valves, specifically the high pressure control valve, we built safety features into the stuffing box. So if you’re doing a trim swap or changing the packing on a high pressure control valve, as you’re, unthreading the stuffing box, which has the packing, trim and everything attached to it, as you’re backing that out, there’s a weep hole that’s drilled into the body of the valve that before you completely unthread the stuffing box, any pressure that’s trapped in the body would start weeping out that little hole.

And that alerts you that, Hey, there’s still pressure. The pressure will eventually bleed out and it should stop hissing. If it doesn’t, then that means you’ve got a leak somewhere, in a ball valve or somewhere upstream or downstream. So that’s specifically what that feature is for.

Curtis: So if I’ve unthreaded to that point and hear it coming out of the weep hole, give it a second to see if that was just trapped pressure? 

Kyle: Right. Give it a second. See if it slows down or stops completely. You don’t want to do anything else to the valve until it stops completely. If it doesn’t ever stop, then that means you’ve got a leak somewhere and you need to take care of it before your work continues.

Curtis: Any stories or near misses that you’ve come across when this procedure was not followed? 

Kyle: I don’t have any personal experiences with that. I’ve heard some horror stories of people working on a valve. One near miss that I heard of was, somebody was working on a high pressure control valve and the specific trim size that they were using was eighth inch trim. On that trim the ball on the stem is smaller than the shaft of the stem itself, so with the topworks off of the valve, it’s able to come out of the stuffing box. There is nothing keeping it in.

So if there is pressure trapped in the valve body it can push that stem out of the stuffing box. So, this individual was taking off the top works. He was going to work on the valve and the stem shot out of the stuffing box. 

Curtis: Was he over it?

Kyle: No, his face wasn’t over the valve at the time, but if it would’ve been, he would’ve definitely been injured. So, in that scenario, I don’t know what he did up to that point, but definitely he wasn’t aware that there was pressure on the valve body. He probably did not double block and bleed the pressure of the body. 

It is a really important safety procedure. The more you’re around these things the more comfortable you get, and sometimes you neglect some fundamentals. If you’re out in the field and there’s pressure to get work done or you’re trying to go fast to get to the next thing—keep in mind that it only takes one mistake to change your life or somebody else’s life. So take the time to make sure you’re doing it right.

If you ever feel uncomfortable with the situation, whether it’s something you’re doing or something that somebody else is doing, speak up. Safety is always number one. We want everybody to go home safe. 

Curtis: Kyle, thanks for covering this really important subject. So, make sure you’re following the double block and bleed procedure whenever you’re working on a valve or doing any kind of maintenance on your equipment. 

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