Dear Goga,
1. The Tank has no gas blanketing as per what you have said.
2. If you have a certain amount of fluid exerting a certain amount of pressure in the vessel beyond an acceptable threshold, then we need to relieve the fluid. For this purpose we install PRVs. The limitation of PRVs is that they operate unidirectionally. If you set a certain pressure for it to open, it opens at that pressure, and that's about it, it cannot be used to relieve reverse pressure (Vacuum).
3. This is where PVRVs are being used, because they have two-way capability. This means that they have two set-points, one Pressure set point and a vacuum set point.
4. When you have two set points, the valve has bidirectional functionality. If the pressure in the tank becomes too high, the Pressure SP is triggered and relief occurs by expelling fluid from tank, and if the pressure in the tank becomes too low, then the Vaccum SP is triggered and fluid enters the vessel from outside to relieve the vacuum.
5. Now, when we have a pressure vessel with multiple relief valves, the main purpose is redundancy, and to reduce the load on each PVRV. By putting a single relief valve, you might not find a suitable relief valve orifice size for relief of very high flow loads. In these cases, the relief load is distributed by using multiple relief valves. This way we reduce loads, and costs for PVRV. Also, redundancy is ensured, which is an important pre-requisite in many process safety measures.
6. In your case, you have two PVRVs with SPs (76, 19 mmHg and 127,38 mmHg). So if there is a relief requirement, it gets relieved by the 1st PVRV. If the load is too high for it to handle alone, then the 2nd PVRV comes into play and relieves fluid further. We are not waiting for the fluid to reach higher pressure. We are in fact giving a provision for relief at lower pressures itself, so that going to the higher pressure can be avoided unless it is of very high relief load (worst case), where even the higher SP valve will have to open up for relief.
7. You can have PVRVs with same set-points. But the disadvantage of this is as Fallah has mentioned.
Valve chattering is very likely to occur. Look at it this way, when you have a pressure vessel with a certain amount of fluid pressure, then we want the PVRV to open at a certain set point only. But when we have two PVRVs at the same set-point, there will be a condition where each of the PVRVs opens alternately and relieves little amount of fluid before the other one interrupts in between and starts opening too because of overlap in their operating ranges. This process occurs recursively and will lead to unregulated, rapid and alternate continuous opening and closing of both the valves, which is known as Valve chatter. Valve chatter can damage the valve, and is noisy and easily avoidable.
8. In order to avoid this valve chatter, it is advisable to maintain the relief set points of each PVRV in an operational range not coinciding with other PVRVs, because we only require one to function at a time and only need the other one to open once the initial PVRV is fully loaded and not otherwise.
Regards,
Shantanu Kallakuri
Edited by shantanuk100, 06 April 2016 - 04:38 AM.