SH Steam leak in sensing leg
SH Steam leak in sensing leg
(OP)
We have a small steam leak in the upstream sensing leg of our steam export (750°F, 640 psig) billing meter, causing it to read low. It is before the isolation valve, and appears to be fragile (the leak is under insulation), so it will not be repaired until an outage. I'm drawing a blank, is there a way I can determine the effect of a steam leak on the downstream static pressure in the sensing leg? I have calculated the flux through a potential leak, I just need to figure out how that will affect the pressure at the transmitter, so I can determine the approximate leak size and be able to calculate a corrected flow.
Thanks,
Thanks,





RE: SH Steam leak in sensing leg
Is the line vapour all the way, or are you expecting part of it to be condensate filled? Steam blowing through condensate would make a significant difference. A layout setch might help.
Matt
RE: SH Steam leak in sensing leg
Here's a sketch of what is happening.
RE: SH Steam leak in sensing leg
Re your concern :
Would guess you've got a venturi meter here (as opposed to an orifice plate) to minimise pressure drop at design flow.
Because this is a low dp device, small errors in measured dp make a big contribution to the readout error. It is anyone's guess what the contribution of this steam leak would be to the measured dp at the dp cell.
Can we replace this venturi with an orifice that gives you more dp at design flow? Then error contribution to final flow readout originating from this leak would be minimised.
Alternatively, use a new venturi meter that produces a higher dp at design flow?
Reranging the dp cell for the higher dp range can be done in your E/I workshop.
The higher the dp produced by the new venturi or orifice plate, the smaller the error % would be due to this steam leak.
RE: SH Steam leak in sensing leg
If the sketch above shows the exact arrangement of tap points and instrument tubing you will likely have condensate accumulated in sensing legs at all times, and this will lead to a continuously erroneous meter readings. In the current setup, it seems that both steam and condensate are blown through the hole, and if this is the case, it is virtually impossible to perform any accurate estimation of the corrected flow.
I'd suggest to assess this arrangement with Instrument Engineer and change the layout of connections, separators and tubing for the flow transmitter, if deemed as required. Bottom connections are not a good practice. Spirax Sarco has excellent guidelines: http://www2.spiraxsarco.com/resources/steam-engine...
Dejan IVANOVIC
Process Engineer, MSChE
RE: SH Steam leak in sensing leg
RE: SH Steam leak in sensing leg
If these are side taps on the venturi meter - okay - what of the lines out of the condensate pot to the dp cell - are they from the top of the pot and continously rising to the dp cell ? - If not, the previously mentioned concern still remains.
If as you say the pots are level with the taps, and the sensing lines off the pot continue from off the top of the pots, then liquids accumulating in the pots would self drain back into the main line via the venturi body.
If you want to avoid a shutdown to replace this meter, may be talk to a hot tap specialist and see what they can do about this ? One big risk with hot taps is not here - flammable hydrocarbons, so that is a big plus.
RE: SH Steam leak in sensing leg
Can't you just remove the insulation and put a clamp over the leak hole, and survive till the next turnaround? It is more effective than any mathematics one may come up with. Just my 2 cents.
Dejan IVANOVIC
Process Engineer, MSChE
RE: SH Steam leak in sensing leg