design temperature and check valve 5

[poli60]

I have an injection system for 2 gases.
aux line is 4" with design temperature of 100°C, operating pressure at 10 barg; main line is 24", Tdes = 300°C, Pop = 3 barg.
the mixing is realized with a special holed distributor at the end of the smaller line that enters in the main tube at 90°.
just before the mixing device, a check valve is fitted on the 4" line; the control valve is upstream.
on the aux line, the Tdes of 300°C has been limited to the check valve.
IS IT RIGHT/SAFE?
thanks and kind regards

 

[LittleInch]

Hmmmm, In general you can't rely on check valve to hold for more than 10% of max flow. Given the significant discrepancy in temperature, I think your 4" line is in some danger and should be at the same DT.

Having said that, given the pressure is very low, what can your 4" pipe achieve at 300C? Even with a big de-rating, I would have thought it could hold 3 bar.

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

 

[SNORGY]

What are the operating temperatures relative to these respective design temperatures, and what upset conditions give rise to the design temperatures? The transmission of heat against the flow through the 4" line, which operates at 7 bar higher pressure, *I suspect* will be principally by conduction as long as there is flow. Without knowing the flow rates involved on both sides, I can't be sure, but the scenario can be simulated. Simultaneously with conditions of no flow in the 4", is there still flow in the 24" line? If not, then again, I imagine that the primary transmission of heat into the 4" line would be by conduction. Otherwise, in a misdirected flow case I agree with LittleInch and would further add that you might even need 2 check valves in series to limit the flow you can hold against to 10% - are you mandated to apply API-521 in this case if the concern is overpressure or reaching an unacceptable combination of P and T due to back flow? LittleInch alludes to that as well.

If the main concern is the line getting hot, I don't see a check valve as a safeguard, nor two in series for that matter. Are both lines insulated here? I'd put in an ESD valve right at the check valve. But the bigger issue might be the piping system configuration itself. If the 24" line gets that much hotter than the 4" line, then anywhere it grows, it's taking the 4" line with it. The overall piping flexibility needs to be assessed as well as the heat transfer effects.

At the end of all that being said, I agree with poli60 that this needs to be looked at, probably by all of Process, Mechanical, Piping and, to some extent, Instrumentation disciplines. On first inspection, it doesn't appear to be "safe", but it depends on what the operating drivers are that give rise to the mismatch in conditions.

 

[curtis2004]

poli60,

Although pressure difference for mixing two streams seems not out of ordinary, as others mentioned above, this should be looked at with different people, possible even formal HAZOP study shall be conducted.
There is also another potential of over-pressure of 24" line from 4" with 10 barg pressure. I don't think that your 24" lines is designed to 10 barg.

Thanks,
Curtis

 

[georgeverghese]

When there is no flow in the 4inch line injection line, the fittings on this line which are downstream of the last isolation valve will heat up to the temp of the main line - so these fittings ( including the last isolation valve) should all be good for 300degC.

 

[EmmanuelTop]

While definitely not the best practice installation, I would say you are on the safe side in case the following applies:

1) Pressure in the auxiliary line is always higher than pressure in the main line - this eliminates the possibility of backflow of hot stream into the auxiliary line. Check possible scenarios that can develop for this system: normal operation, prolonged shutdown, failure of auxiliary pump (if applicable), etc. and see if any of them comes with a possibility that high temperature materials enters into lower design temperature section.
2) I don't know what material is used for the auxiliary line and which piping code is followed, but with design conditions of 10 barg and 100 degC there is a good chance that you are still OK with 3 barg and 300 degC. Check pressure rating of the auxiliary line material at 300 degC.

In case you come up with definite conclusion that the design conditions of the auxiliary line can be exceeded, you can install another (dissimilar) check valve in series with the existing one (also rated for 300 degC), and/or add an actuated shutdown valve that will isolate the auxiliary line in case the design conditions are exceeded. This valve may cause the need for additional overpressure protection of the auxiliary line, depending on the way the operating pressure is developed in the auxiliary line (e.g. reciprocating pump etc.).


Dejan IVANOVIC
Process Engineer, MSChE

 

[SNORGY]

EmmanuelTop,

Is it your interpretation or industry adopted good practice for the check valves to be dissimilar? I agree with it being a good idea, but while, for example, API RP 521 mentions that it's something that should be considered, it doesn't explicitly mandate it. I (we) see this frequently in our design reviews and HAZOPs, and where I work I am wondering if we should align more closely with that approach, despite the client push-back that we invariably get.

 

[EmmanuelTop]

@ SNORGY: You are correct when saying there are no international codes or standards that prescribe the use of two dissimilar check valves as mandatory. Some companies that I worked for in the past have this requirement specified based on preliminary analysis of possible consequences (in the form of a checklist). In some other companies I have seen just the type of NRV being specified depending on service conditions, but not the number of NRV in series. It really varies from case to case. On many occasions, installation of a single check valve is more than sufficient.

The underlying reasons for employing two dissimilar valves are based on the fact that two dissimilar NRV's have lower number of common failure modes. That is the first argument. The second is based on the extent of the consequences of reversed flow (e.g. overpressurization, exceeding design temperature, contamination, corrosion, creation of a hazardous mixture etc.). Ultimately there is no - and there shouldn't be in my opinion - prescriptive approach for applying two NRV's in series. That is probably why it is not mandatory in standards. in many cases it would be unnecessary overkill, and in some other it still wouldn't provide sufficient degree of protection.

Dejan IVANOVIC
Process Engineer, MSChE

 

[poli60]

Thanks to all of you for the valuable highlights/advices. I'd like to have people like you in the HAZOP team ;-)
@LittleInch - I share your point about the extension of the 300°C to the auxiliary line (up to the block valve and including it!)
@SNORGY - Your analysis is very accurate. I'm also worried about the big deltaT and the piping configuration.
@curtis2004 - The potential overpresure of the 24" line merits attention. I don't thonk it's normally considered in systems like the present one (ie oxygen enrichment in FCC catalyst regeneration) but now, since the air line is new, I'll push to have PSV on it.
@EmmanuelTop - I'm used to see the double check valve applied where the differential pressure is above 20 bar. And, in this case, where oxygen is involved, it's better to minimize the number of fittings (and it's difficult to propose an alternative type of check valve).
Kind regards