Hi all,

A question on the physics behind rapid pressurization (relatively high pressures e.g. 300 bar) and perhaps transients in systems. For simplicity, imagine that we only have a piece of pipe with an initially closed valve on one side and a pressure indicator on the other side. The current pressure in the pipe is atmospheric (~1 bar, zero over pressure). Further, on the other side of the valve I have connected a high pressure line. If I open and within very short time close the valve, the pressure indicator would first quickly rise and indicate same pressure as in the high pressure line. Just after closing the valve, the pressure indicator drops to a value less than the pressure in the HP line.

I the valve would remain open for a longer period of time before being closed, the effect can not be observed.

Could someone explain the physics behind this? Do temperatur variations during the pressurization have to do with it?

Many thanks!

C

 

CCG,

Is the pressure gauge you are using not filled with oil (something that is commonly done to reduce gauge pressure can reading fluctuations?

Is it possible to throttle the valve (presuming there is a valve) between the gauge and the pipe, as sort of an orifice, to try to reduce pressure fluctuation indications?

In my mind's eye, what's happening is the gauge is reacting to the sudden pressurization by over-indicating because the mechanism isn't damped in any way. The maximum pressure you are seeing (because it doesn't sound like you have any method of recording the pressure) is probably just the indication which is the result of sudden movement of the gauge mechanism (a Bourdon tube???) when pressure is very suddenly admitted and then the flow is stopped when you close the valve of the pressure source and the gauge falls back to the actual pressure in the pipe--which didn't have time to build up to actually equalize to the same pressure as upstream of the pressure source valve.

The size of the valve (internal opening) and the size of the connection of the valve to the pipe being pressurized--as well as the size of the pipe upstream of the valve you are using to admit pressure to the pipe can all have an effect on the rate at which pressure builds up in the pipe. If the connections and/or the valve internal dimension are smaller, they are acting as an orifice--on the high pressure source. Yes, there will be a temperature drop, especially in the beginning--but without data it's very difficult to say how much of an effect it has or would have.

Many times the way something is being measured--and controlled--has an effect on the measurements. An undamped gauge; the type of gauge mechanism; the sizes of the pressure taps/lines the gauge is connected to; the presence--or absence--of an orifice in the sensing line of the gauge; the ability of the pressure source to quickly fill the volume of the pipe to raise its pressure quickly (in other words, the ability for the high-pressure source to flow the medium into the volume being filled; all of these things can have unintended effects (which can be the result of more than one condition!) on the measurements/readings being taken.

Sorry if you were looking for maths and formulae to substantiate the explanation. That's not my department. I have spent a great deal of time trying to gather data for analysis in such a way as to minimize the effects of measurement methods, and sometimes the size of a sensing line or the type of valve used in the sensing line or the position of the sensing line valve's plug or the type of gauge/transducer can have some unexpected affect on the readings being gathered--and when those readings are being used to judge the performance of rotating equipment versus guarantee those effects can be the difference between pass or fail--success or litigation. Perhaps not the same as whatever you are trying to measure for whatever purpose you are trying to quantify and understand the "results" you are seeing (which seems like anecdotal data at this point--which means it's the result of visual observations and not recorded values). Since we don't really understand what you're trying to do--measure or control the pressure inside a volume which might be subject to sudden change.?.?.? we probably can't be of much more help.

 

CCG,

The location of the pressure tap for the pressure gauge could also play a part in the readings you are reporting. And, again, the volume of high-pressure (air?) being admitted when you open the manual valve from the high-pressure source is also probably a large factor in the readings you are seeing.

 

https://en.wikipedia.org/wiki/Boyle's_law

 

Curious_One,

Is your post implying that the temperature change (caused by the flow of high-pressure into an area of lower pressure) is responsible for the decrease in pressure after the valve is closed to stop the flow of high-pressure into the pipe?

 

CSA,

The poster wanted some math. I figured the relationship between pressure, volume, and temperature is as good a place to start as any. Although the original post did not describe much, I figured temperature must be a constant.

Since the gauge is located close to the valve, I am "guessing" that the exspansion of the gas into an area of larger volume and at a lower pressure that the gauge is sort of like measuring the high side of an orifice.

Once the pressure equalized into this unknown volume the effect will/has deminished. If the pipe described is venting to atmosphere, the orifice effect is definitely in play. Just guessing from a very limited description of what is trying to be obtained.

 

Curious_One,

Actually, I'm re-thinking my responses. There's just too much we don't know about the configuration, volumes, sizes of valves, pressure taps, control valve type, etc. There's almost nothing we know for sure, except the pressure (300 barg). We don't know what temperature the high-pressure medium is at.

Actually, I was just thinking digitally, and trying to reason through this without too much maths since there's so much unknown.

The original poster said if the valve controlling the high-pressure medium flowing into the pipe were left open longer that "... the effect cannot be observed." To me that says if more high-pressure medium is allowed to flow into the pipe that the pressure differential across the control valve drops to near zero and the gauge reading doesn'r drop but goes to very near the same pressure as the high-pressure medium--but that depends on how long the valve is open and how much high-pressure medium flows into the pipe.

I don't think this is too much about maths or temperature differentials--that's my personal opinion. I think it's a curious effect--but it's really not very complicated. It depends on a lot of things--not the least of which is the type of high-pressure medium AND the size of the control valve and connection between the high-pressure source and the pipe, including the volume of high-pressure medium which is available.

There's just too many unknowns.

I have been wrong in the past.

I will be wrong in the future.

I may be wrong about my response to this thread.

I'm flexible.

I'm done with contributing to this thread. I will continue to follow it, but there's just too many unknowns. This is all speculation.

 

CSA,

I will use this to rare moment to remind you of the maths (you already know) involved with inlet bleed heat. First, the valve flow coeffs of the valve itself depending on position of the IBH valve are known (adjustable orifice).

The inlet and outlet bleed heat pressure transmitters are located at least 5 pipe diameters from the valve.

Therefore, math may be used to calculate actual bleed heat flow. I do not need to know the math.

However, I do need to know how it works and why valve flow coeffs are know and why the pressure transmitter are AT LEAST 5 pipe diameters away from the valve.

Perhaps, the original poster will discover why we do not put a pressure gauge next to a valve. Also the relationships of pressure, volume and temperature.

 

I know maths; I hesitate to always use it to explain phenomena when I don't know whom I'm communicating with. (Think explainig VArs beginning with vectors and angles....)

But, thanks for the reminder.

 

You said VArs......Although it has been talked about big time on control.com, I plan to bring up "real world issues" and I hope you and Phil Corso will jump in. I will start the thread when I "line up my ducks"

 

Phil Corso’s your VAr man on this forum. (Hope you like nebulous questions for answers.)