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If a pressure source has not been specified anywhere within the system model, or if the only pressure source is isolated from the rest of the system, the "No Pressure Sources" warning will be returned when the system is calculated.

"No Pressure Source" warning displayed in the Message Window in v12 and above.

"No Pressure Source" message displayed in the Warning dialog box in v2009 and before.

In order to calculate a system and avoid this warning at least one pressure source must be specified in the model.  Possible pressure source devices include:

  • For PIPE-FLO Compressible:  Pressure Source
  • For PIPE-FLO Professional, Stock, and Flow of Fluids:  Tank, Boundary pressure or Spray Demand

Without a pressure source in the system the hydraulic calculations cannot be solved.  For example, if we consider the Bernoulli equation and the simple system shown below it is easy to see that the system cannot be solved without at least knowing the pressure at the Tank or at the Demand.  To solve the Bernoulli equation for this system we must know six of the seven parameters.  In this system we are fixing the flow rate which allows us to determine the velocity (v1 and v2) and head loss (HL) terms in the Bernoulli equation and knowing the system elevations we can define Z1 and Z2.  With all other terms defined, there are infinite values of P1 and P2 that will satisfy the equation.  We must define either the inlet (P1) or outlet (P2) pressure in the system (i.e. the pressure at the Tank or at the Demand) in order to solve the Bernoulli equation.    

\bigg(\frac{P_1}{\rho}\bigg)+\bigg(\frac{v_1^2}{2g}\bigg)+(Z_1) = \bigg(\frac{P_2}{\rho}\bigg)+\bigg(\frac{v_2^2}{2g}\bigg)+(Z_2)+HL

This concept is straightforward in an open loop system, but the most common situation in which a pressure source is neglected is with a closed loop system.  Perhaps this is because the fluid is being recirculated through the system without necessarily ever entering or leaving a tank or some other pressure vessel.  However, closed systems always have some type of pressure source involved whether it be an expansion tank, accumulator, or other similar device.  It is therefore necessary to include this equipment in the system model in order to satisfy Bernoulli's equation and perform the hydraulic calculations.

Another common misconception is that a pump satisfies the requirement to have a pressure source in the system.  A pump is merely a device that adds energy to the system in the form of pressure.  In order to calculate how the pump will operate in the system the program must be able to determine the dynamic resistance of the system and in the case of an open loop system, the static resistance in the system as well.  In addition, to determine the actual suction and discharge pressures at the pump and the pressures throughout the system there must be a pressure source to start from. 

In the closed loop system shown below the pump is adding 156.2 ft of head or 67.59 psi to the system at the pump discharge.  If this system had no pressure source specified there would be no way to determine what the actual pressure would be at the pump suction and discharge.  In the first case the system has a pressurized expansion tank set to 20 psi g giving an equivalent suction pressure and with the pump contributing an additional 67.59 psi the discharge pressure comes to 87.59 psi g.  If the expansion tank pressure is increased to 30 psi g, the pump discharge pressure also increases by 10 psi to 97.59 psi g, as shown in the second case.  Despite the change in system pressure in accordance with the increased expansion tank pressure, the pump operation remains the same at a 156.2 ft of head or 67.59 psi.    Without the pressure source modeled in the system, in this example an expansion tank, there is no way to determine what the actual system pressures are, you can only determine the dynamic resistance or how much energy the pump must add to the system.  Knowing the actual pressures in the system is essential to determining the presence of vacuum conditions, cavitation, flashing, and exceeded equipment design limitations.  

 

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