PRV Installation Issue 2

[DVICTOR1982]

Hi Gentlemen,

I have a question related to PRV Inlet Piping. I have to install a relief valve on the steam supply pipe from the steam turbine tapping to a header rated at 11.3 barg. We have two parallel steam paths from turbine that converge and run as a single pipe to join the steam header. We intend to install the relief valve in the section bewteen the converging point and the main header. As per BS EN ISO 4126-1:2004, the recommended distance between any converging, diverging or bend and relief valve should be 8-10 nominal pipe diameter. However, we are unable to find sufficient horizontal pipe run to install the Relief Valve. Therefore our mechanical contractor has proposed that the pipe be run vertically for 8-10 nominal diameter and the RV installed at the bend on a straight tee with the branch leading to the steam flow to site.

However, I have few concerns about the proposed location:

1. There is a chance of chatter because of disturbed flow pattern at bends.
2. Damage to valve seat – Because of centrifugal action, heavier particle such as rust and water droplet will tend to separate out from the flow stream at the bend and will impinge on the valve seat causing damage and leakage.

Can somebody please advice if the proposed location of safety valve is fine and whether it complies by the guidelines say API 520 Part 2 or BS EN ISO 4126 - Part I. Any guidance would be much appreciated.

 

[don1980]

The intent of the API and ISO4126 guidance is to ensure the PRV is located sufficently downstream of anything that causes flow disturbances, which create pressure fluctuations. This is primarily important when the PRV set pressure is close to the operating pressure because, in those cases, pressure fluctuations may cause the PRV to simmer or pop open.

I would locate the PRV on a straight run (10 diameters distance). The location shown in the sketch is at a sharp bend (tee), and since the PRV inlet connection is the straight run of the tee, it will definately act as a partical trap. Water droplets, however, are not a concern because this steam will have plenty of superheat since its coming from a turbine extraction port. I suggest you work with the pipe designer to find another layout - one that allows the PRV to be positioned on a straight section of pipe, 8-10 diameters from a flow disturbance.

 

[georgeverghese]

a) Inlet piping to the PSV should allow steam condensate to drain back to a low point which is continously drained out.
b) Why have we got the PSV downstream of these double block valves? - this would be acceptable only if the block valves were fully rated for the steam supply design pressure to the turbine - standard design practice does not permit locating PSVs' after equipment maintenance isolation block valves, even if they were designated as locked open.

 

[DVICTOR1982]

Thanks @ georgeverghese & Don1980.

Your replies are very useful. I am trying to re-route the pipe so that I have around 10 dia stratight run before the Relief Valve. However, I am interested to know whether there is any guidance that restricts the current proposed location or is the proposed location compliant with the code. I will have a reference basis to advice the mechanical contractor against the proposed location.

@ georgeverghese - We have a steam trap installed beneath the vertical 1200 mm section (through mudpot). The RV is there for a very different reason. The downstream header is rated at 11.3 barg, while upstream we have steam turbine extraction and a steam generator. When steam turbine is not running, the steam will be supplied by the steam generator. The steam generator operating pressure is designed for 10 barg, however the RV on steam gen is set at 12 barg (to avoid RV pop up due to pressure spikes during unstabkle firing). That is why the RV is located downstream at that point to protect the downstream header (which is rated only upto 11.3 barg).

Thanks once again.

 

[don1980]

Refer to API 520 Pt II (6th ed.) section 4 & 5 for recommendations related to installation. Note that this is guidance - it's not requirements. Also, understand that codes (e.g. ASME Sec VIII) leave most details to the user, including those we're discussing. So, explain this to the mechanical contractor in terms of good engineering practices rather than saying one way is right (required) and the other is wrong (disallowed).

 

[georgeverghese]

Okay, got that on the relief concept.

Strange logic: if you previously had the steam gen RV deliberately set at 12barg in order to avoid chattering, we now have the new RV set at 11.3barg - so the chattering concern will be applicable to this RV now.

It would make sense if the 12barg RV is a conventional type RV (90% of set pressure to simmer), and the new one is a pilot operated (99% of set press to simmer). Even then, wouldnt it make more sense to replace the current 12barg RV on the backup steam generator with a pilot operated one set at 11.3barg?

 

[DVICTOR1982]

Thanks @ Georgeverghese & don1980.

@GeorgeVerghese - Its bit strange, I agree but our situation was like this -

1. We already had a system in place with WSE (written scheme of examination) at MAWP -11.3 barg. The site operating pressure is at 9.5-10 barg.
2. We had to remove one of the existing boiler and install steam generator in place of the existing boiler. There was also steam available at 9.5 -10 barg from the backpressure steam turbine.
3. The new steam generator operating pressure will be 9.5 barg, however its RV is set at 12 barg.

There is signficant pipe run between steam generator and existing header, so the pressure spikes are very likely to subside before they reach RV. The Steam Generator is 10 t/h, and is coil type and not a shell-tube boiler. The SG does not have any steam space like normal shell-tube boiler, and has just a small steam separator. Further, it has internal mechanism to stop firing, when steam pressure reaches 11 barg. So its very very unlikely, the RV in question will ever see that pressure.

The manufacturer was reluctant to decrease the set point of the RV on SG to 11.3 barg. Therefore to comply by WSE, we have installed a RV on the steam supply line from SG to existing header.

 

[georgeverghese]

Got that

Is the 12barg RV a conventional type? If it is, it will begin to simmer at approx 10.8barg.

A pilot operated one, set at 11.3barg, would simmer at above 11barg, so this gives you a little more room on pressure excursions than a conventional one set at 12barg. Getting this manufacturer to understand this takes away all the construction risks involved with installing this new 11.3barg RV further downstream.

 

[georgeverghese]

Does your company or local authority prohibit the use of pilot operated RVs' in steam service?