Two weeks ago I introduced my list of the top 5 common causes of pressure control issues on tanks and we are currently in the process of identifying a solution (or solutions) for each problem.
To recap the top 5 common causes of pressure control issues on tanks—in no particular order. (Click the links below review the solutions)
1) Variable or incorrect regulator inlet pressure
2) Stored product causing plugging
3) Overlapping or “tight” setpoints
4) Lack of self-draining sensing lines or low point drains
5) Lack of continual maintenance programs
#3 Overlapping or “Tight” Setpoints
The installation of multiple pressure vacuum relief vents (PVRVs), also known as conservation vents, is a common practice by many companies. This is primarily being done by end users wanting to eliminate a single point of failure or to satisfy the emergency fire case in lieu of an emergency relief vent. The mistake occurs when a single PVRV is sized for normal tank breathing and the additional PVRVs are at the same setpoint.
When multiple PVRVs are set to the same setpoint, all of the PVRVs will open slightly off of their respective seats as the tank pressure is relieved. This can cause two problems. The first is excessive wear on the redundant PVRVs. As with any valve as it opens and closes the seating surface will wear over time. The second problem is known as flutter, or oscillation, inside the valve chamber. When vapors are relieved close to the set pressure, the pallet can flutter on the seat leading to premature valve wear or worse, the pallet will not reseat properly.
One way to avoid these potential problems is by staggering the PVRV setpoints. When possible, I recommend designating a “normal breather vent” to handle the normal day-to-day tank in-breathing and out-breathing. The normal breather vent will have setpoints lower than the other PVRVs on the tank. The remaining PVRVs are at setpoints high enough to not open unless there is an abnormal event that requires additional pressure relief capacity.
By staggering the setpoints, excessive wear and potential premature failure of the PVRVs is avoided, reducing overall maintenance. In addition, the PVRVs are kept out of the “flutter zone,” reducing the chances of excess emissions due to the pallet not properly reseating.
If you have any additional comments about staggering PVRV setpoints or PVRV flutter please comment below.