How to size relief valves downstream of the (flowmeter) regulators?

[Engineer_M]

Generally we use Cv value of the regulator to size the relief valves. But not sure how to size relief valves for flow meter regulators since it comes with a preset pressure. Any help on difference between a flowmeter regulator and a standard pressure regulator would be very helpful. Is Cv value not applicable to flowmeter regulators since they come with a preset pressure? Does the orifice size remain constant?

Any help will be greatly appreciated, thanks!

 

[georgeverghese]

Most regulators are sized with Cv at around 40-70% of the max wide open Cv of the regulator. The downstream RV should be sized with both of the following conditions being coincident: (a) max or wide open Cv of the failed open regulator (b) pressure upstream of the regulator at max normal.

 

[RVAmeche]

I would take the max Cv of the regulator (or maybe the max potential Cv, if the internals are capable of being swapped). Make a flow model with the worst case upstream pressure and see what you get downstream the flowmeter regulator, then size your PSV per usual.

 

[Engineer_M]

Thank you for your response @georgeverghese.

So we use the Cv value and pressure upstream of the regulator for our calculations. But the flowmeter regulators that I have come across don't typically come with the CV value information. I am wondering if its because they come with a preset pressure and the orifice size remains constant? Any tips on how to size a relief valve for these types of regulators?

 

[LittleInch]

I really don't understand what these regulators are doing or why you don't have the CV of them when full open.

Are they part of the flow meter package?

They really are just control valves. You should be able to find the values you need from whoever you bought them from in the data pack.

They fact they control on a set pressure is not relevant. It's a control valve. It has a CV when fully open. The vendor can tell you what this is.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

As @LI says, you may find the max wide open Cv data for these regulators from vendor literature. Else call up the vendor. All regulators will have this information.
For the pressure downstream of the regulator during this failure scenario, to the relieving pressure at the RV, add the built up frictional losses going back up to the regulator exit. So there is some element of trial and error involved here, since the built up losses are directly related to the mass flow at the time of failure.
If you are not familiar with these calcs, ask for a senior process engineer to assist.

 

[Engineer_M]

Here is the regulator information:

https://store.cyberweld.com/smduflre30se.html

Most regulators have this information readily available, I was just wondering if anyone else came across something like this.

 

[gerhardl]

Your product information requires some comments, as it is not obvious from your first post that you are talking about small scale valves for welding gases. Details on existing piping and safety devices are still missing.

1. The gas flask/tank will necessary need/have/include a safety valve.
2. Where is the regulator placed in the pipeline? I presume the high-pressure side of different gases already have sufficient safety devices.
3. I also presume that the flow to the regulator is controlled by one, or possibly two needle valves in parallell.
4. All posted answers to you are still valid.
5. Your practical problem is then to find the total cross-section area that restricts the flow to the regulator. The factory (supplier) will normally help you here with suggested equipment range. The supply can anyway not be larger than the full opening of the needle valves regulating input amount. Needle valves for regulating have normally actual flow borings far below piping diameter.

 

[LittleInch]

I went hunting the miller website, but I think these regulators are just too small for them to bother with data like this. But you can just call them and ask them....

What relief valves are you using for this??

And why?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Engineer_M]

Thank you for your inputs, here is some clarification:

The regulator will be attached to a DOT cylinder to supply Argon gas to the welder. There is no pressure control from the cylinder, the regulator is rated for max inlet of 3000 psig and the supply pressure from the cylinder is below that.

We want to install a relief valve to protect the welder in case the regulator fails. This is to meet our requirements. In this case it’s a little difficult to select relief valves because there is no Cv information (the manufacturer doesn't have it either). There seems to be no way to know if the relief valve flow capacity is sufficient for regulator failure scenario.

 

[gerhardl]

Again: as my previous post: worst case is inlet pipe diameter to the regulator, or if inline regulating inlet valve, the actual (not nominal) diameter of the regulating (needle type?) valve.

 

[Mconlon]

Not sure what the full flow rate of the Miller regulator is. At a minimum you can go with a Cv of 18 based on the 1/4” inside pipe diameter at the inlet of the flow regulator. If you can find a relief valve with a Cv higher than 18, you’ll be covered.

Could also consider adding a needle valve upstream of the flow regulator. Start with the valve in the closed position and open it till the regulator hits the 60scfm mark. Won’t be hard to find a relief valve that exceeds that flow rate.

 

[georgeverghese]

You may be able to extract a max Cv value for this regulator by asking Miller what the minimum supply pressure from the Ar tank should be in order to maintain required max flow at the specified downstream pressure.

 

[Engineer_M]

Thank you everyone, all the responses were very helpful! Installing a needle valve upstream is a good idea, I will look into that.

 

[LittleInch]

but where are the valves in the system to cause a high pressure?

If your isolating valve is up stream of the regulator and this is shielding gas, is there a valve downstream of the regulator to block flow??

Also consider that you have needle valves already upstream of the flow meter to regulate flow.

Even if they were fully open, I suspect they would be your key flow restriction.

But if you've not got a locked in pressure scenario then you don't need relief valves.

Remember relief valves can leak / pass you then need to think about where the gases go as they can kill people if they accumulate in the wrong place.

So you are opening up a potential list of issues here.

Just saying.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.