Pressure control resolution 4

[rollingcloud]

This is for pressure testing on a pressure relief valve. The aim is to adjust the inlet pressure from 0.00 to 150.00 psig. However, we are experiencing jumps in the pressure readings, typically in increments of 5-10 psig—for example, jumping directly from 110 to 120 psig. Ideally, the pressure should increase in finer increments, like 111, 112, 113, etc. Our testing setup, as designed by a previous engineer, utilizes the TESCOM 44-1800 series. This regulator is capable of handling a maximum inlet pressure of 6000 psig and an outlet pressure range of 0-2500 psig. Since we are only operating within a 0–150 psig range, the 0-2500 psig capability appears to be excessive to me. Perhaps a regulator of the same type but designed for a lower pressure range would provide better resolution/control?

The current setup from our supplier includes a pressure regulator on top of the tank and a needle valve before the inlet. However, this arrangement is resulting in pressure jumps of 5-10 psig during testing when the operator adjusts the pressure regulator's knob.

 

[3DDave]

Stiction may be preventing movement in a continuous manner. If in doubt add a very long rigidly attached lever. This will overcome the rubbery response of skin and tendon that may exaggerate stiction as well as giving a multiplication to the observed motion.

 

[LittleInch]

I'm amazed that in a regulator sized for 0 to 1500 you can get pressure steps of 10 psi, never mind 1 psi.

Your regulator is probably worn out at the lower end and any minute bit of dirt or wear will result in jumps of a few psi at a time.

Talk to the regulator supplier to get a more suited device.

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

 

[mk3223]

One thought is to route the second line with a smaller regulator and use for fine tuning with a small increment for the pressure adjustment.

 

[rollingcloud]

Thank you all for the ideas. I will try to add a longer lever and find another regulator.
The one in the picture (supplier) looks like it's for 0 - 6000 psi, so probably even less resolution.
Is pressure-reducing & shutoff valve same as pressure regulator? I got confused by all the different names.

 

[LittleInch]

It can get complicated, but in general a Pressure reducing valve is just the mechanical valve element. Shut off valve usually means an on/off isolation valve, and regulator takes feedback from the upstream or downstream pressure and "regulates" the pressure regardless of flow so long as it is within the range of the regulator including no flow.

But check your understanding is the same as the person you're talking to....

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

 

[Prometheus21]

Hi,

As with any piston sensed pressure regulators you are going to have a bad time, especially when they get older as the sensing elements have a tendency to "lag" due to aging and lack of lubrication.

"Our testing setup, as designed by a previous engineer, utilizes the TESCOM 44-1800 series. This regulator is capable of handling a maximum inlet pressure of 6000 psig and an outlet pressure range of 0-2500 psig. Since we are only operating within a 0–150 psig range, the 0-2500 psig capability appears to be excessive to me" - Correct. With those small pressures and fine increments you should select a pressure regulator more suited for a lower inlet pressure and working pressure.

I would recommend the following:

1. Select a pressure regulator that has a maximum inlet pressure of 3000-3500 psi (Europe usually 230 bar) and a working pressure (WP) of 0-150 psi (0-10 bar). This is a common pressure range and is used extensively. With your wish for a 1 psi increment however, and the fact that you are using this to pressure test a pressure relief valve, I would recommend installing either an electronic pressure gauge or manual pressure gauge with a very high classification on the low pressure side (WP side).

2. You should also ask the supplier of the regulator to switch out the adjusting screw under the bonnet for a fine-threaded one. This is typical for applications such as yours where precise pressure readings are needed, and the fine-threads allow for a much higher resolution compared to your typical regulator.

3. "Is pressure-reducing & shutoff valve same as pressure regulator? I got confused by all the different names." Pressure regulator, pressure reduction valve, reduction valve, pressure reducing valve are all names for the same thing. It is however not a shutoff valve per se, but some do have that as an added configuration on the outlet, effectively making it one. In day-to-day nomenclature however those are not referred to as a shutoff valve, rather a pressure regulator.

Good luck!

 

[rollingcloud]

Thanks again LittleInch, but you lost me on the last sentence...

 

[rollingcloud]

Hi Prometheus21,
Thank you for such a detailed response; it’s very informative. You brought up another great point. The test plan does list digital pressure gauges with Grade 3A, 0.25% accuracy; however, one of the gauges displays to the tenth, while the other displays to the thousandth. I will add this to the task list.

If the supplier switches out the screw for a fine-threaded one, I assume the mating/female thread in the regulator would also need to be replaced, correct?

 

[Prometheus21]

"The test plan does list digital pressure gauges with Grade 3A, 0.25% accuracy; however, one of the gauges displays to the tenth, while the other displays to the thousandth. I will add this to the task list." Yeah that makes a lot of sense. Usually we deal with class 1 or 1.6 in the industrial gas industry for day-to-day testing and manufacturing of pressure regulators and other types of equipment. However, in cases where customers need to "set" the pressure on a pressure relief valve they require the type of regulator that you describe. The setup varies of course, but since we deal mostly in bar over here we use a minimum of 2 decimals for laboratory precision. As a digression; I remember designing a one-stage H2 pressure regulator for 4500 psi inlet pressure with WP of 2.9-10 psi, with 1.5 psi increments. It worked great in the end, but it took a lot of time as the customer refused to use a two-stage.

"If the supplier switches out the screw for a fine-threaded one, I assume the mating/female thread in the regulator would also need to be replaced, correct?" - If you mean the inlet/outlet threads then incorrect. The performance of the regulator with regards to resolution is independent of the inlet/outlet connection, the flowrate on the other hand is very dependent on the inlet/outlet configuration. (Simplified explanation here)

Below you can see a typical sketch on the layout of a TESCOM 44-1800 series regulator. Under the control knob you have a screw (marked in red circle). To increase the resolution of the regulator this screw is often changed to a fine-threaded version (along with the threads in the bonnet casing). Another way is to replace the spring below it (Both would result in the regulator requiring more turns of the control knob to achieve the desired working pressure (allowing for fine-tuning)). As a regulator operates on a balancing-of-forces principle there is many ways to achieve the same result, and I'm not saying these are the only ways to do it; just the way I would do it in most cases.

If I were you I would contact the valve supplier (or another well-renowned one like Swagelok) and list your required specs. They will most likely be able to help you out and give you a standard of the shelf regulator that would work for you.

 

[Prometheus21]

I just re-read your question again, and realised we were talking about the same female mating thread (Bonnet). YES you are correct.
Sorry, it's late and it has been a long day

 

[rollingcloud]

Thanks for another through response!
My bad, I didn't even realize there are threads in the inlet & outlet, otherwise I would have been clearer. I was not sure if the female thread is part of the bonnet or an integral part of the housing.

 

[Prometheus21]

As you can see from the section view of the regulator above (zoomed image below), there are many threads in such a regulator. The bonnet is attached to the main unit by threads, The adjusting screw under the control knob has male threads that mates with the female threads in the bonnet. One way to increase the resolution of the regulator is therefore to switch out the bonnet and the adjusting screw, allowing for finer threads.

 

[rollingcloud]

Understood, thanks for the section view. How do you disassemble the TESCOM 44-1800 regulator knob? Since I dont have the ease of access to the supplier, I am thinking to just purchase a new regulator.
Swagelok's high flow, manual gas pressure regulator looks like a good fit. However, I dont see the pipe connection size (1/4 NPT).
Link

Is digital flow meter a good choice if I want 2 decimal precision? Currently its a rotameter, with .5 increments.

 

[Prometheus21]

"Understood, thanks for the section view. How do you disassemble the TESCOM 44-1800 regulator knob? " - I can't speak for this particular brand, but usually there is a self locking plate in the control knob to prevent to prevent the user from going over the set pressure (Can be "chipped out" , others you simply screw out). Considering you dont have ease of access to the supplier I agree with your thinking - buy a new regulator.

So on this HF series the following specs are given:

■ Compact size with flow rates up to 200 std L/min
■ Maximum inlet pressures up to 500 psig (34.4 bar)
■ Manually adjustable outlet pressure up to 150 psig (10.3 bar)
■ 316L VIM-VAR stainless steel body for ultrahigh-purity applications
■ 1/4 in. VCR® metal face seal; 1/4 in. tube butt weld; 1.5 in. and 1.125 in.
modular surface-mount end connections

It does however look like this series uses butt weld nipples and and vcr glads to connect inlet and outlet respectively. If this is a problem then your best bet would be to reach out the Swagelok directly and lay out your requirements: Test medium, pressure ratings (inlet, outlet, required increments), operating temperatures, inlet and outlet connection points (often a broad selection available upon request) ++.

Do NOT order a digital flow meter! You need a digital pressure gauge to set fine increments of pressure, not to measure the flow. If you want simplicity I would connect a Lex1 or similar digital pressure gauge. The pressure range is 0-1000 bar, 0.05 % accuracy with a 5 digit LCD display (3 decimals). Any such digital pressure gauge would work fine for your application if you can connect it to the outlet line.

 

[rollingcloud]

Oops, I forgot to mention that the flowmeter is for leakage test. We typically do pressure test first and then leakage test.
But yea, for the new pressure regulator, it looks like I have to buy more pressure gauges since they dont come with it.
I found more suitable ones on their website: Link
Since I already have a big pressure regulator on top of the cylinder, I think either dual stage or single stage would work as the new regulator.
I am reaching out to Swagelok for more info on the exact model. It's great that you introduced this brand, they are pretty close to where I live.

 

[Prometheus21]

Hello again,

I finally got around to respond to your latest answer.

Question: If you have a digital pressure gauge then you will easily detect any leakage as a pressure drop, a flowmeter AND a pressure gauge therefore seem redundant? When my company test a pressure regulator we use oil-free air at 1.15 times the nominal cylinder pressure it is intended for. After 15 minutes the regulator is approved if no leakage through the valve or to the atmosphere is detected (and the tester locks the regulator so that the customer cant overadjust the working pressure by accident - (similar to setting the relief pressure on a relief valve).

Remember, if you already have a regulator on top of the cylinder and you add another single stage regulator further downstream you effectively have a two-stage regulator from that point Two single-stage regulators will always be better than a bad two-stage regulator (which unfortunately describes most of what is available on the market today).

 

[rollingcloud]

This relief valve is somewhat odd; it's unlike any other relief valves I have seen on the internet, not sure what type of relief valve it is. When the pressure is normal or low, the flow passes through port A of the valve and into port B (which is connected to a regulator in the actual operation). When the set pressure is reached, port A closes, and air from the regulator is relieved through the valve’s port C.

The leakage test is part of the acceptance test. The pressure gauges are connected to ports A and B for testing the closing and opening pressures. When testing for leakage, the flow meter is connected to port C to check for external leaks, and to port B for internal leaks.

There are also a few performance issues that I am hoping to solve, but since its very different topic, I will start a new thread.

 

[rollingcloud]

What does the venting option do for you in the pressure regulator? not sure if I need it.

 

[Prometheus21]

Understood, thank you for the sketch. I have been in the industrial gas industry for a while and one thing is for sure there is always another valve

I have attached a document from Swagelok regarding the venting options. Looking at your configuration I don't see you needing one as you already use port C to vent.