Pressure measurement in gas piping

[RGME]

All,
Can anyone recommend a good source of information for designing/locating pressure taps in an existing gas piping system?
I am trying to locate a tap for pressure control purposes in a 12" gas line (MAOP = 800psig). Unfortunately there is not much straight length of pipe available in the area where measurement is required. Ideally I would be looking to install the tap with 10 straight upstream pipe diameters. I am trying to determine if the 5 ft of straight pipe available will provide reliable pressure readings, and if not, how inaccurate the readings will be.
Thanks.

 

[khan101]

If you are not measuring differential pressure, do you still need 10 pipe dia straight run?

 

[zdas04]

Measuring a static pressure is a bit metaphysical. The pressure at the wall will vary from the pressure in the center of the pipe, but an instrument that can sense the pressure in the center of the pipe will change that pressure through vortex shedding. I've seen several studies and simulations that all say that for virtually any real-world application, the difference is not within the accuracy of any calculations you are likely to do with the data.

The reason for the straight-pipe requirements in square-edged-orifice gas-measurement is the assumption in the equations that the flow is "fully developed" prior to going through a known restriction. If the flow is swirling (for example) then it will tend to corkscrew through the orifice and get a significantly reduced dP for a given mass flow rate.

Measuring pressure does not have the same requirements. In fact, I will always prefferentially take a pressure reading in a dead leg above a gas flow (this keeps condensation from accumulating in the instrument).

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The Plural of "anecdote" is not "data"

 

[jsummerfield]

Pressure is generally easy unless you after mm H2O levels of pressure. Typically the instrument connection is on the top of pipe. Provide a 3/4-inch or so connection with a block valve.

The instrument should be above the tap in pressure, especially if the lfuid is condensible. (If the pressure is fairly high, perhaps about 500 psig, then the potential head impact for a measurement a few feet below the tap is not such an issue.) If you were measuring steam it would be different. Steam may exceed the rating of the instrument. We typically come off the side or lower connection or if from the top add a siphon accessory to trap condensate thus isolate the high temperature from the instrument.

As previously stated, the upstream diameter requirements apply to flow meters, especially head type flow meters.

John

 

[m777182]

I've read about flow rectifirers that shorten the 10 straight upstream pipe diameters requirement.Search the web or look at ISO 5167-1:1995 page 8.
m777182

 

[bmsg]

You do not need any straight length for pressure measurement. Take a pressure tapping (1/2" or 3/4")at any location which is convenient for you. Take the taing from any portion of the 12" pipe ABOVE the horizontal plane through the centre, so that the taping points downwards into the pipe (and any moisture or condensate falls back into the pipe).

 

[jsummerfield]

Diameters up and down not applicable to pressure. Several flow meers avoid such requirements. Find a flow conditioner thread at

http://www.eng-tips.com/viewthread.cfm?qid=77288&page=1

John