31 replies to this topic

[icasensio]

Hi all,

I have to define several TRV in some pipes (6") and I have several questions:

1.- Fluids inside the pipe could be (not simultaneously, but at different times) natural gas, lpg and naphta, (40 barg and 12ºC). Do the valve work correctly if I define a common 3/4" TRV?
2.- Valves will discharge in a common line, wich will be connected to the flare. How can I define de diameter if I don´t calculate the flow rate?
3.- I think LPG will produce flashing and I don´t know if I have to consider it.

Many thanks in advance.

[Art Montemayor]

Ines:

I presume that you are in the process of preparing the “Data Sheets” for some thermal relief valves (TRVs), prior to purchasing them. The subject of thermal relief valves has been aired and discussed many times on this Forum in the past. (You can read these threads by using our SEARCH engine) Many engineers have labored in the past with misconceptions of how to calculate, install, and depend on these relief devices. Please allow me to cover some very important points regarding the specification of TRVs. You may already be aware or know perfectly well what they are and how to “size” them, but I mention this for the benefit of not only this discussion, but also for the benefit of any young process engineers that may be reading this thread now and in the future:

It is vitally important to know that the design and use of TRVs incorporates a very specific and narrow Scope of Work (alcance). These relief devices are meant to protect piping and equipment from failure (rupture) due to HYDRAULIC PRESSURE caused by the liquid internal fluid expanding inside the piping or equipment. The hydraulic pressure created is due to temperature increases in the fluid that may arise due to exposure to radiant solar heating or near-by heat sources. In order for this to occur, the piping and equipment in question MUST BE 100% LIQUID FILLED – which is the normal case with liquid piping. That is ALL that these relief devices are supposed to do.

If there are TRVs on a liquid pipeline that may be used for other services – especially gas, vapor, or 2-phase flow – it is very important that this be noted on the related P&ID and, if possible, on the TRV itself. These devices are very simple in construction and meant to simply “pop” open instantly and similarly shut once the hydraulic pressure has been relieved. They are not designed to maintain themselves open and to adjust to any variance in flow or pressure. That is why there is no need to calculate the liquid flow rate going through them when they open. (Note that I wrote “liquid” flow rate – not gas flow rate) The total amount of expansion liquid that is required to relieve the system is very small (measure in a couple of cm³) and doesn’t merit calculations. Remember, this is HYDRAULIC pressur e – not gas (or compressible fluid) pressure. Because of the quick and instantaneous action of this type of valve, the subsequent seating of the valve may – or may not – be effectively 100%. Additionally, keep in mind that the seating of this valve is considered important to prevent liquids from leaking or seeping out through the seat. This is very important when the valve’s discharge is piped directly to a flare system and any leakage can’t be visually detected. Therefore, some of these TRVs are designed with “soft” (Teflon, Nylon) seats in order to ensure a tight, leak-proof, seat. That is why it is very important to note this in a Data Sheet when it is known that different fluids (especially hydrocarbons and gases) will be used in the same service. Some soft seats may be attacked by the different fluids and this will defeat the seating ability of the valve.

Remember that this is a relief device that reacts to high pressure – NOT TEMPERATURE. Some engineers have the erroneous impression that a thermal relief valve reacts to temperature. This is an erroneous concept. What this means is that the valve will respond to high gas pressure as well as to high liquid hydraulic pressure. HOWEVER, this type of relief valve cannot be relied upon to discharge the proper amount of excess gas or vapor that may be needed – for example, in a fire case. Recall what I stated above: these valves have a narrow scope of work and if there are TRVs on a liquid pipeline that may be used for other services – especially gas, vapor, or 2-phase flow – it is very important that this be noted on the related P&ID and, if possible, on the TRV itself. As a responsible engineer it is very important that you correctly identify what this valve protects against so that a false sense of security is not assumed.

The correct answer to your first question (Do the valve work correctly if I define a common 3/4" TRV?) is: Yes, the TRV will work correctly with different liquids that fill the pipe 100%. BUT, it cannot be expected to “work correctly” when in gas service. It isn’t designed for that.

The answer to your second question (How can I define de diameter if I don´t calculate the flow rate?) is: you define the diameter as either ½” or ¾” size simply because – as I previously have stated – the amount of thermally expanded liquid is insignificant (although the generated hydraulic pressure is very high!).

Your third query (I think LPG will produce flashing and I don´t know if I have to consider it) has importance to it. You are correct; LPG will flash into a cold mixture of liquid and vapor. I would not worry about the amount of vapor if you employ the bigger TRV size (3/4”), but I would be concerned about the type of seat and materials used due to the cold temperature generated. I would clearly indicate this possibility on the Data Sheet when specifying the TRV. This is the problem when managers try to save money by using common pipelines for different fluids. You may – or may not – have problems.

I hope this response will help you in your work effort and I apologize for the lengthy discussion and explanations.

Saludos

[fallah]

icasensio,

Art explaned almost all issues regarding the TSV installation on a piping system, then i would like to add some points as follows:

- As per API 521 sec 5.14.4, a TRV might not be needed to protect a piping system from thermal expansion if there would be one of four conditions (a/b/c/d) in that section.
- When vapor production due to flashing of fluid is passing through a TRV, let say passing LPG, would be possible then reduction of mass flow capacity of the TRV due to choking could be a point to be investigated.
- Because the TRV's are liquid relieving devices and have normally higher blowdown (around 20% or even higher), then especially for those connected to flare network, care should be taken having adequate margin between maximum operating pressure of piping system and TRV set point.

Fallah

[icasensio]

@Art

Many thanks for your explanation. I will consider your recommendation indicating the different fluids in Data Sheets.

By the other hand, I have other TRV for the same cases and its size is bigger than 3/4" ( actually 1 1/2"). Discharge line is 6" and I need calculate the flow rate at discharge to define the tipe of valve (convenctional, balanced or pilot)

Edited by icasensio, 22 June 2012 - 04:48 AM.

[icasensio]

@Fallah

Your answer has been so helpfull. From the first point I guess it is not necesary to consider natural gas.

I have applied the procedure D.2.2 in order to size the TRV for LPG, ( this size would apply to naphta case also) but I have had to stop because volumetric flow rate is needed. Could I use the formula posted by Julysan?

Thanks in advance

Edited by icasensio, 22 June 2012 - 04:53 AM.

[Art Montemayor]

Ines:

I forgot to mention one important point that may interest you in attacking your problem.

You may - or may not - have already considered the application of expansion chambers to replace a reliance on TRVs. What I refer to is the use of an expansion chamber segregated from the process fluid by a stainless steel diaphragm. The expansion chamber is filled with nitrogen and reacts to the thermal, hydraulic process liquid expansion by compressing the nitrogen. This device eliminates the need to discharge to atmosphere or a flare in order to protect the pipeline. This could be a viable alternative (and attractive) solution to your multi-service pipeline.

Saludos.

[kushal.raval]

@Art

 

I like the idea of providing expansion chamber. I have the similar issue of frequent TRV releases in multiproduct pipeline. At the pump suction there are connections of individual products (viz. MS, Naphtha, SKO, ATF, HSD etc) which are normally blocked expect one. TRV popping up is reported frequently. Can you explain further regarding provision of expansion chamber individually on these lines. Please pass on if any literature is available.

 

Thanks