Discussion on the allowable pressure drop and maximum closing pressure difference of control valves

The allowable pressure drop of a control valve or on-off valve is the maximum pressure loss that the valve can withstand under normal operating conditions, ensuring that it can regulate flow properly. The maximum closing pressure difference refers to the greatest pressure difference that the valve can withstand when it is closed; an excessive pressure difference may cause damage to the seals or to the valve itself. In pump systems, the maximum discharge pressure typically experienced by control valves or on-off valves is the pressure at the pump outlet during operation. When the pump slows down and stops or operates at a low flow rate, the system pressure decreases. At such times, the control valves or on/off valves should be able to function properly and not become disabled, as the maximum closing pressure difference taken into account during design is based on the pump’s maximum operating pressure. The statement that the maximum allowable pressure difference for control valves should be greater than the maximum closing pressure difference is intended to ensure that the valves can function properly under all operating conditions, including proper regulation when flow rates are high and safe closure when flow rates are low or the valve is closed, thereby preventing damage to the valves due to excessive pressure. .

There is a slight misunderstanding regarding the concept of maximum shut-off pressure; it is calculated as the maximum pressure that could occur in the upstream system (usually the set pressure of the safety valve) plus 1.2 times the pump’s outlet head. The factor of 1.2 is based on experience, and it is typically derived from the pump’s flow characteristic curve; 1.3 can also be used as a more conservative value. Thereafter, based on the maximum shut-off pressure, the operating temperature under that condition is determined; this value is then compared with the design pressure of the pump volute and the pipelines. If all three values are greater than the pump’s maximum shut-off pressure, the requirements are met. If any of these values are lower or close to that threshold, cost considerations come into play. For example, if the downstream pipelines are very long and raising the pressure level throughout them would result in high costs, a safety valve is installed at the pump outlet, with its setting set at 95% of the lowest of those three values. The maximum allowable pressure drop, again, depends on the pump’s performance curve; the head generated by the pump varies as flow rate increases – the head decreases when the flow rate is high. Moreover, a higher flow rate also increases the pressure drop across the control valve. Therefore, to ensure that the pump maintains its basic hydraulic performance, an internal limit value is set for the pressure drop across the control valve, which is the maximum allowable pressure drop. From an instrumentation perspective, the diameter of a control valve is generally 0 to 2 sizes smaller than that of the pipeline. For example, for a pipeline with a DN100 diameter, the size of the control valve is usually 100 or 80, with a minimum of 50. This is done to provide the control valve with a certain degree of operational flexibility, so that the valve opening remains between 25% and 80% under normal operating conditions. If the maximum allowable pressure drop calculated in practice is too low, the diameter of the control valve can be increased accordingly to reduce the pressure drop.

When a pump operates at low flow rates, shouldn’t its corresponding head be close to the maximum value?

If there is a strong wind today, or if someone is pressing against the door, it will be difficult to open it (as it opens outward); the back pressure is high, so more force is needed to open it, which corresponds to the torque required of the valve. The maximum shut-off pressure difference is used for this purpose. After the door is opened, in order to allow a certain number of people to pass through it, it is necessary to find an appropriate opening angle for the door. During the process of adjusting this angle, there is a drop in pressure at the door; to ensure that enough people can enter, the door area cannot be too cramped, so it is important to control the size of the door.

The definition of the maximum shut-off pressure remains the same, as the pump’s maximum shut-off pressure Pc,max is equal to Ps,max + 1.2ΔP. Here, Ps,max is the sum of the discharge pressure of the container’s safety valve and the highest possible liquid column pressure in the container; this value takes into account severe operating conditions. What I’m looking at here is a switch valve, namely a shut-off valve located before the pump’s outlet. I’m not familiar with the specific parameters of the container. The pump in question is a pneumatic diaphragm pump, with a maximum discharge pressure of 2.1 barg and a normal discharge pressure of 1.65 barg, resulting in a pressure difference of 2 barg. The maximum shut-off pressure achieved by the XV valve is approximately 4 barg, which seems to be consistent with that figure； Furthermore, I used some example calculations; the allowable pressure difference for the control valve downstream of the pump came out to be only a few dozen kilopascals, while the pressure downstream of the pump can reach several hundred kilopascals. I can’t understand this

So what can the allowable pressure difference of a valve be used to determine? Why don’t we use the maximum closing pressure difference directly when calculating the pressure loss in a piping system? The maximum shut-off pressure difference is often much greater than the pressure drop when the control valve is in operation

There is a contradiction here: for piping systems, the allowable pressure drop of the control valve is part of the head required by the pump selected. When the control valve is placed after the pump, its maximum closing pressure difference equals the maximum pressure in the upstream system. A head of 1.2 times that value is already greater than the allowable pressure drop; how is it possible to ensure that the allowable pressure drop is greater than the maximum closing pressure difference?

Question: That’s my understanding. For pump systems, control valves or on-off valves are generally installed after the pump, to link the pump and/or the instruments of the end equipment. As shutdown approaches, is it this valve that has to withstand the maximum discharge pressure of the pump (i.e., the maximum closing pressure)?
Answer: The maximum discharge pressure has nothing to do with the pump’s performance curve. The additional 1.2 takes into account the highest head that can be achieved according to the pump’s performance curve, plus the setting pressure of the safety valve in the upstream system. Under normal conditions, the safety valve does not activate. Therefore, as shutdown approaches, before closing the pump outlet valve, it is necessary to switch to the minimum flow circuit before stopping; at that point, the pressure exerted on the valve remains within acceptable limits, as the selection of the valve took into account both the safety valve and 120% of the required head, ensuring that the valve meets all design requirements. Here, the maximum pressure that the valve experiences when you stop the vehicle is only at a point to the left on the characteristic curve; it is not possible to reach the maximum discharge pressure, and the safety valve does not even activate. Question: Since the maximum shut-off pressure difference calculated at this point is much greater than the allowable pressure drop of the valve as determined when I calculated the pipeline system, does this mean that when the pump is about to stop or is operating at a low flow rate, this control valve or on/off valve becomes ineffective? Answer: The maximum shut-off pressure difference is the pressure difference between the two sides of the valve when it is fully closed. The allowable pressure drop for the valve is the pressure drop that can be generated by the valve during hydraulic calculations. Generally, the higher this value, the better the operational flexibility of the valve; however, if the pressure drop is too large, it requires a higher pump head, and the same applies to the piping used. Additionally, flashing may occur at the valve outlet. There is no conceptual relationship between the maximum closing pressure difference and the allowable pressure drop of the valve. I don’t understand why it’s paralyzed – is it because the valve can’t be closed? The valve is pneumatically controlled and can be operated. Question: Why is it said that the maximum allowable pressure drop for control valves must be greater than the maximum closing pressure difference? (Picture shows the calculation of the maximum discharge pressure of the mechanical pump) Answer: I haven’t heard of such a claim. The maximum allowable pressure drop is used in hydraulic calculations; it determines the overall pressure drop across the system, after which the head capacity of a pump can be determined. The maximum shut-off pressure is used to determine the pressure capacity of the pump volute, fittings, and pipelines, in order to assess whether the pressure rating is appropriate and whether an additional safety valve is needed after the pump. There is no connection between the two