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What does a negative (-) dP calculation mean for a pump or a control valve in PIPE-FLO or Flow of Fluids?

by Engineered Software, Inc.

Using equations such as the Conservation of Mass and Energy, you have to define what direction the energy flows positive. In PIPE-FLO® and Flow of Fluids, when a device is adding energy to the system, such as a pump, the differential pressure (discharge - suction pressure) is shown as positive number. 

If adding energy is positive, then when a device is removing energy from the system, such as a control valve, the differential pressure (outlet - inlet pressure) is assumed negative by definition.  Because of this assumption, PIPE-FLO displays the control valve pressure drop as a positive number, with the understanding that the device removes energy from the system by default. This article discusses the meaning of the calculated results when a sizing or centrifugal pump or a sizing or control valve has a negative dP which is just the opposite of the normal sign convention. In all the conditions, the devices are set to design flow rate.

 

Negative dP for Pumps 

Starting in PIPE-FLO v12, the Total Head and dP of a pump are displayed as positive numbers when the pump is adding energy to the fluid and negative when energy lost from the fluid.  Shown below are pumps with a negative dP calculation due to an inlet pressure set to 100 psig and the outlet set to 0 psig. Instead of a pressure gain across the pump, there is a pressure drop.

In this case, the sizing pump and the centrifugal pump (VFD operation) are set to 800 gpm and the calculated dP is -71.92 psi. Both cases indicate that the fluid loses energy as it flows across the pump, which is not the normal condition for a pump.

Negative dP for Control Valves

For control valves (and any other device in which head loss occurs across the device), head loss is positive and results in a pressure drop across the valve. If the network solution results in a pressure gain across the valve, then head loss will be negative. Since this is physically impossible for a valve (valves don't add energy to the fluid), the results indicate the system as modeled is not realistic.


In this case, the sizing valve and the control valve are both set to 500 gpm and the dP is -6.27 psi.

PIPE-FLO Professional will display the following messages when a negative dP occurs with a pump and control valve.

When PIPE-FLO calculates a negative dP with a sizing pump or centrifugal pump (set to VFD operation), it means it is behaving as a control valve. This is illustrated by removing the device and seeing how much flow rate occurs without it.

Shown above, a flow rate of 1528 gpm can be achieved without a pump. So in this example, trying to set a pump at any flow rate less than 1528 gpm, will cause the pump to act like a control valve. The resulting negative dP is how much energy has to be removed from the system.

When PIPE-FLO calculates a negative dP with a sizing valve or a control valve set to a flow rate, that means it is behaving like a pump. This is illustrated also by removing the device and seeing how much flow rate occurs without it.

In this screen shot, a flow rate of 327.6 gpm is only available in the pipeline. A valve setting of 500 gpm cannot be achieved due to a lack of energy in the system. The negative dP that PIPE-FLO calculates is the amount of energy needed to attain the desired flow rate.

In summary, when a negative dP is calculated in PIPE-FLO, the results are physically impossible to obtain. A negative dP with pump means the pump is acting as a control valve and a negative dP on a control valve, means the control valve is behaving like a pump.

Attachments:

neg dP.pipe

 

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