SH Steam leak in sensing leg

[bigsteve80]

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,

 

[mbt22]

Upstream or downstream? Instead of reading the line pressure it is reading the line pressure minus the pressure drop due to flow to the leak. I'd be surprised if that effect was significant though.

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

 

[bigsteve80]

Upstream. The sensing legs have condensate from the condensate pots down; the leak is before the condensate pot.
Here's a sketch of what is happening.

 

[georgeverghese]

This arrangement looks odd - would have thought that all sensing lines in condensible vapor service should be remote diaphragm sealed to prevent liquid collection in the sensing lines - else liquid level accumulation in the 2 sensing lines will not be same, leading to errors in measurement.

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.

 

[EmmanuelTop]

I'm not an expert on this subject but I agree with mbt22. I wouldn't expect any difference in readings for properly designed connections, if this is a pinhole-size leak passing steam only.

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-engineering-tutorials/flowmetering.asp

Dejan IVANOVIC
Process Engineer, MSChE

 

[bigsteve80]

I drew it that way out of convenience - the taps are on the side, and the pots are on the level with them.

 

[georgeverghese]

Just realised that these taps are on the body of the venturi meter and not on the export line.

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.

 

[EmmanuelTop]

As discussed above, it seems impossible to perform any decently accurate calculations in case there is condensate blown together with steam.

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

 

[georgeverghese]

pls note that hot taps with steam at these conditions is not without its risks - some operating companies expressly forbid hot taps, so check with plant safety folks if you are thinking of going down this way. The hot tap procedure itself is usually critically reviewed by operating company materials selection engineers also.