Thermal Expansion Valve-Minimum pipe Length/Volume 2

[ENG-RAY]

Hi Everybody,
do you know any criteria for a minimum pipe Length/Volume, for that Thermal expansion relief valve shall be considered? I have seen in some company's Design practice that they consider 0.5 m3 for LPG and for all other medium 1m3 or 30 meter.
It is not always possible to size the valve because it is not always a pure substance and we do not have all parameters.So we need to decide based on volume.On the other hand we have found lots of pipings which have been blocked , are heat traced (there is a difference of 80 deg C between operating temperature and heat tracing temperature) , have no thermal expansion valve but never had problem with thermal expansion. So we do not know how we should decide if we need a thermal expansion valve or not.

 

[Latexman]

Control valves frequently leak enough to relieve thermal expansion, unless one goes for really expensive low leakage control valves.

For piping inside the battery limits of a process, it is usually not a worry, hold-up volume is too small. On outside battery limits lines, especially large diameter and/or long and/or heat traced, I sharpen the pencil on thermal expansion.

Sorry, I do not have a hard criteria.

Good Luck,
Latexman

 

[don1980]

When heated, liquids will expand. If that liquid is in a truly isolated system (fully blocked in), and the temperature rises, then overpressure is going to occur regardless of the volume of liquid in that isolated system. So, that's all you need to focus on (can the temperature rise while the system is isolated) rather than focusing on volume. In small volume systems, such as those in ISBL piping, we generally don't experience failures due to thermal expansion. But that's because the volumetric expansion of a small amount of liquid is a small number, and this small amount of expanded liquid can often escape, even through a "closed" valve. However, it's obviously better to not relay on leaky valves to protect the system. Instead, design the system such that there's an open path for expansion, or implement procedures that prevent the isolation of liquid-filled piping. Alternatively, install a thermal relief valve. If your piping system uses tight-sealing valves, such as plug or ball valves, then you're much more likely to experience overpressure problems.

Thermal PSVs are commonly needed on long pipelines (e.g. OSBL piping) because these high-volume systems generate a higher volume of expanded liquid. In this case, leaky valves often won't save you - their leak rate is too low for the higher volume of expanded liquid.

 

[The Obturator]

API-521 (2020) Section 4.4.12 'Hydraulic Expansion', provides some commentary on the issue of PRD selection for thermal expansion. In particular paragraph 4.4.12.2 'sizing and set pressure' This basically states that a relieving rate is not easy to determine and the relieving rate is small. It goes on to say that generally a 3/4" x 1" TRV is used. If this is doubtful, the section further advises on other methods to determine a relief flow.

*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Sawsan311]

Dear All,

API 521 is stating 3/4'' x 1 '' for TERV as a typical sizing for relatively small systems having 454 Kg (500 L) of liquid blocked in section. However, the sae section 4.4.12.3 is stating that detailed sizing of TERV will be required for larger systems specially long uninsulated aboveground pipeline/piping i.e. roughly > 500 m exposed to thermal volumetric expansion of the liquid (while developed pressure doesnt exceed the vapor pressure).

In such systems, it will be difficult to implement forced operating procedure of draining the liquid before every shutdown or to ensure continuous operation if they are isolatable. other factors such as the difference between the operating T and maximum solar sun radiation as well as presence of any vapor bubble to preclude liquid expansion can be considered while assessing if thermal expansion is required or no.

Regards,

 

[LittleInch]

There are only guides and the practice of the operator to help you with this one.

Some element of practicality needs to happen otherwise you end with hundreds of TRVs all over the place.

Your guidelines above sound reasonable to me, but you need to consider the volume, the amount of possible heat input and which sections are most likely to be isolated using "proper" isolation valves and what the temperature would be when the valves closed.

I've seen systems where there is cascading TRVs, i.e. one section relieves into the next one and so on until it reaches a TRV to reduce the number of closed drain points.

Relying on valves passing isn't a good move. just when you want a valve not to seal 100% that's what it goes and does....

So how you decide? Follow the specification or guide given to you by the owner or come up with one and get it approved or make a judgement call. That's why it's called design.

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

 

[ENG-RAY]

Thanks every body. My problem is that we have a lot of piping which theoretically can be blocked in between two manual valves or one manual and one automatic block valve . The pipes have different length and sizes.If we decide to install a TRV, I am sure 3/4'' x 1 '' will be enough. The only criteria which I have seen is EXXON design criteria which says for 1m3 blocked volume of liquid a TRV shall be considered.The reason why it does not happen for smaller volume is that the pipe itself will expand a little bit as well and if the volume of liquid is less than 1m3 and also the pipe is not too long, the expansion will be enough to avoid over pressure.I have seen myself that even a smaller volume (0.5 m3) of LPG has resulted to over pressure and leakage only because of exposure to sun.I think for Polymer it is completely different.Polymers have flexible linkages and bent bonds and they do not behave like a pure liquid so I guess the minimum length and volume should be more. Do anyone has experience with polymer and can tell me what is the approach in polymer plants for pipe which are heat traced?
It does not seem reasonable to me to install each 30 m one TRV!