Pressure regulating valves were discussed in the preceding blog Valves – Pressure Reducing Valves. Pressure reducing valves, although commonly used where pressure is an issue, are not always the best alternative. The pressure reducing valve has a close cousin called a “Back Pressure Regulating Valve” which monitors and controls pressure in a similar but different way. The back pressure regulating valve is actually a better choice in some instances requiring dynamic control of pressure.
The most common example of a back pressure regulating valve for most of us is the pressure relief valve found on hot water tanks in our homes. In this use, the back pressure regulating valve is a safety device to prevent the hot water tank from exploding if it should overheat. Similar valves are used in nearly any situation requiring that pressure not exceed some safe limit. These safety valves are often one operation devices. Once the valve performs its function, it must be replaced.
The following illustration is a schematic representation of a dynamic back pressure regulating valve.
Pressure on the inlet side of the valve pushes against adjustable spring pressure to open the valve once the desired pressure is exceeded. Once the back pressure on the inlet side is reduced, the valve closes to maintain back pressure on the inlet. In order for the valve to function properly, the back pressure on the outlet side of the valve must aways be less than the desired back pressure on the inlet side. If the outlet pressure temporarily exceeds the back pressure on the inlet side, it will be held back by the valve by holding the valve closed. In this case, however, there is nothing to prevent the back pressure on the inlet side from increasing to equal the back pressure on the outlet side.
One more consideration – – In order for this valve to perform its pressure controlling function, it is assumed (and necessary) that increased flow from the source will result in reduced pressure. Although this is a pretty safe assumption in nearly all cases, there may be a few exceptions – so be careful and know the source characteristics when specifying a valve of this type!
This type of valve has some interesting potential that I can’t recall seeing used in typical automated cleaning machines. Consider the following example.
A series of counter-flowing rinse tanks includes a pump and filter which returns filtered liquid from the first tank to the last tank in the series. Meanwhile, one or more tank(s) require an exit spray. The sprays operate intermittently as required. At any given time, none or all of the spray(s) might be activated. Most systems I’ve seen would accomplish this using a pump for the filter loop with automated valves to activate the exit sprays as required. A separate automated valve might be used to turn off the return to the final tank when spraying functions are operational. Another scenario would use a dedicated pump for each exit spray.
The illustration below shows a simpler and likely less expensive solution using a back pressure regulating valve.
Pressure is maintained in manifold supplying the sprays by the back pressure regulating valve. Depending on the full capacity of the pump and the pipe size of the manifold, all sprays could be used simultaneously with full pressure maintained to all. Excess liquid continues to replenish the rinse tanks. When the sprays are not activated, the full return from the pump is automatically directed to the rinse tank return. An added benefit of this arrangement is that all sprays are supplied with freshly filtered liquid.
I’m sure that all you clever engineers out there can come up with tons more uses for this oft-forgotten, relatively inexpensive and useful valve – the back pressure regulating valve.
– FJF –
3 comments on “Valves – Backpressure Regulating Valves”
I just noticed a typo in my response to you that may cause confusion. In the sentence – –
The backpressure valve may be “set” to a specific pressure but it would make more sense to have it just compare the pressures on the feed pump outlet and the dosing pump outlet and NOW allow backflow into the dosing pump.
The word “now” should be NOT.
I’m sorry for any confusion this may have caused. FJF
Thank you for your question! Although I am not totally familiar with your application, it would seem to me that the purpose of the backpressure valve on the discharge of the dosing pump would be to prevent backflow in the case that the dosing pump pressure is not sufficient to overcome the pressure in the discharge from the feed pump. In that instance, the discharge pressure of the feed pump would cause a backflow of liquid back through the dosing pump which would not be a good thing. The backpressure valve may be “set” to a specific pressure but it would make more sense to have it just compare the pressures on the feed pump outlet and the dosing pump outlet and now allow backflow into the dosing pump. It would seem that this could also be accomplished using a “check valve” but there may be a reason why the outlet of the dosing pump should be restricted in the case that there is insufficient pressure at the outlet of the feed pump. As I said, I am not an expert on systems of this type and am making a lot of assumptions in the above answer. It would be best for you to check with the designer of the system and request an explanation of the purpose of each feature of the system. This is not an unreasonable request. If you have a schematic diagram of the system and can send it to me, I may be able to give you a better response. John Fuchs
I am designing a dosing system for centrifuge in sludge dewatering application. The dosing pump will pump Polymer solution into the feed pump discharge line at the inlet of centrifuge. The dosing pump has back pressure relief valve. When I asked the vendor regarding the details, they have answered that for avoid overfeeding / siphoning of the liquid a minimum differential pressure is required for proper pump check valve operation for which Back pressure relief valve is provided.
What I understand by this is that the back pressure value is “set” to a certain value so that if the pressure in the discharge pipe line falls below the set pressure the check valve closes. Does this occur, when the pump is not in operation and the check valve needs to close so that required pressure is given by the back pressure relief valve ?
Also there is optional provision of relief valve and anit-siphon valve assembly? I am not sure about their functions