A pump can be added to simulate flow in a model.  Power, efficiency and the pump duty point are calculated and provided in the EDIT box.

Edit Box

Displays a form allowing pump, compresor and fixed flows and pressures to be applied to a pipe.  If a pump is present, a pump curve graph and duty will be displayed after simulation showing pump performance in a simulated model pipe.  If no curve is used for this pump, a blank window will be displayed.


Fig. 3.2.5.1: On the left is a pipe edit box for various applications to a pipe. On the right is a graph depicting pump performance of a pipe that has been simulated.

Fixed Flow

Forces the simulation to produce a flow equivalent to the value entered in the box.  Fixed flows can either be used to simulate the effect of a pump or a valve, or for forcing flow into parts of a model to reproduce observed flows.  A fixed flow cannot be set when a pump is present, and a pump will be removed if the fixed flow is checked.  To remove the influence of the fixed flow, simply uncheck the fixed flow check box.  When a fixed flow is simulated, the program will calculate a required pressure and power to produce the fixed flow.  Where the fixed flow is less than what would normally be simulated, the fixed flow acts as a ‘resistance’ or negative pressure influence, and restricts flow. In both cases, the results of fixing a flow will be displayed in the INFO box.

A fixed flow will force Pumpsim™ to calculate a pressure required to induce flow to the set amount.  This pressure can be substantial if the flow needs to be pushed through a high resistance.  Conversely, the pressure may actually be negative, if the fixed flow forces the flow to be lower than would otherwise simulate, effectively resulting in the fixed flow acting as a higher resistance.

Hint:  If the fix flow is working with compressed air the flow will be free air volume flow.

In all cases, the results of a fixed flow can be observed from the Edit Box information function, which describes the pressure, power and resistance required to produce the flow.  This can be directly used to estimate a pump duty point which would be required to produce the same results.  This fixed pressure can be equivalent to Pump Static Pressure or Pump Total Pressure, depending on the simulation type and location of the pump.

In general, unless estimating the requirements for a pump, the use of fixed flows to reproduce observable flow is generally discouraged at it may adversely affect other parts of a model and does not provide a realistic behaviour for changes in model systems.  In many cases it can mask real problems with the model construction.

Recommend Pump

If a fixed flow is present and has been simulated.  The program will examine the pressure and flow required to achieve the flow, and then search the pumps in the program database for a suitable matching and the RPM to achieve that flow.  The program will consider up to 4 pumps as being a viable match, and display them in descending match order. It will also display the number of RPM necessary to achieve this flow as well as the efficiency of the pump at that point.  To swap the fixed flow for a recommended pump, select the desired pump from the list, the RPM will also be automatically adjusted.

Estimating Pump Pressure Requirements

Fixed Flows are often used to help estimate required pump pressures.  The pressure given for fixed flows will be an estimate of the Collar Total Pressure – ie the total pressure of the flow in the pipe directly beneath the pump.   For a pump with an equal diameter to the pipe, this equates to the Pump Head requirement, however caution should be used for this interpretation because differing pump diameters will alter pump head requirements.  A pump manufacturer will normally be able to utilise the collar total pressure value to an equivalent pump head.  The Pump Head requirement will be a function of the discharge size and velocity of the pump, as well as any resistance and shock losses in the pump and structure between inlet and outlet.  Therefore the pump manufacturer will need to take this into account when selecting a suitable pump.  Also note that the simulated pressures provided will be influenced by the simulated pump or Pipe Discharge diameter (evasé) so ensure this is noted and specified when quoting the duty point summary.

It is important to note that fixed flows contribute to model power consumption, in much the same way as pumps.  The fixed power within a model is summarised from the Run Summary menu item.

When calculating power for a fixed flow, the default pump/fix efficiency from the Settings menu is used.

More information on fixed flow and pressure is available from the Edit Box functions.

Pump Density

Finally, when specifying a pump duty point based on a fixed flow, it is important that the density the fixed flow was simulated in is noted.  Pump performance varies substantially in different densities and a pump manufacturers will need to know the required density so the pump curve can be adjusted for the local condition accordingly. In the simulation, the density is set by the fluid type in the pipe.

Fixed Pressure

Forces the pipe to produce a constant positive pressure equivalent to the entered value in the box.  A fixed pressure cannot be set when a pump is present, and a pump will be removed if Fix Pressure is checked.  To remove the influence of a fixed pressure, simply uncheck the Fix Pressure check box.  When a fixed pressure is simulated, the program will calculate the required waterflow and power to produce the fixed pressure.  The results of fixing pressure will be displayed in the INFO box.

As with fixed flows, fixed pressures consume power, which again is summarised in the Edit Box information summary, or from the Model Summary menu item.

When calculating power and heat for a fixed pressure, the default pump/fix efficiency from the Settings menu is used.

Pump Name, Number and Configuration

Selects a pump. The upper pull down menu allows a pump to be selected from the pump database.  Pumps are sorted in the order established in the Preset form.  Adjacent to the pump name is the number of pumps to be included in the model, and below this is the configuration of the pumps (parallel or series).  Pumps installed in series will increase available pump installation pressure according to the number of pumps available, while pumps installed in parallel will increase available flow.

Pump Database

Open the pump database screen to view or adjust pump data.  Note that any pump changes will not be reflected in the EDIT box until after a simulation.  Also note that modifying the pump database will change ALL pumps of the same name in other locations in the model.  To create a custom change only for the pump at one location, you will need to create a duplicate of the pump in the database, and only modify the duplicate.

Hint:  Pump operating density is an important specification for designing pump types and installations.  Higher water densities will increase the available operating pressure curve (and power draw) for the pump, while lower water densities will have the reverse effect.  This can be an important consideration when selecting or designing a pump for a particular area in a site.  Note that for Pumpsim™ , water densities are assumed fixed at all locations in a site.

Pump Curve

Pump Duty Point (BLUE lines on Pump Curve Graph)

The pump duty point defines the pressure and flow that a pump operates at, and is shown by blue lines on the pump curve.  Note that either an individual pump duty can be selected, or if multiple pumps are present at the installation in parallel or series configuration, the entire combined installation duty can be displayed.   Due to local water density adjustments, the pump curve may have been adjusted to the one defined in the pump database.

The efficiency returned reflects the efficiency curve entered with the pump.  The efficiency curve is the shaft efficiency of the pump.  It does not include electric motor or drive inefficiencies, which are considered separately in the Settings menu under Motor Efficiency.

Pumps Stalled, Low Pressure or Negated

If pumps are forced by other model conditions to run beyond the limits of the pump curve, a warning showing stalled, low pressure or negated will appear during model simulation.

Stalled pumps occur when the pump pressure build-up is greater than the capacity of the pump curve to accommodate.  Pumpsim™ reduces the flow quantity to a point where the pump pressure is no more than the maximum pressure specified in the pump curve.

Low Pressure pumps occur when a pump pressure falls below the lowest pressure point of the pump curve, but is still operating above a zero pressure.  This may occur when a pump is run with too little resistance or the pump is a high pressure type, being used in a low pressure application.  The pump pressure curve in this case may not extend down to the point the pump is operating at.  This is undesirable as the program must make assumptions as to what duty point the pump is actually running at.  In addition the pump is unlikely to be running efficiently at this duty point.  To prevent this warning, simply extend the pump curve to a lower pressure point.

Negated pumps occur when the pump offers no useful pressure to the system, and may even retard flow that would otherwise flow more freely without the pump.  This may be a result of other model factors or pumps forcing or drawing water through the pump.  

When Pumpsim™ encounters a low pressure or negated pump situation, it applies a resistance for flow through the pump above the highest curve quantity.  This mimics real life performance of pumps and effectively restricts flow moving through a pump above its limit.   The induced resistance results in an additional pressure drop and in negated situations, the pump may apply a nominal negative or ‘resisting’ pressure, effectively acting as a ‘brake’ or orifice for the flow.

Any of these situations are not desirable, as pumps are not designed to run beyond their pump curves.  Without a pump curve Pumpsim™ is forced to approximate pump performance and power consumption.  While stalled, low pressure and negated warnings can be ignored, every effort should be made to reduce or eliminate these occurrences within a model.

Pump Pressure Curve

The pump pressure curve associated with the pump (or pumps if combined is selected).  Note that the pump curve may have been adjusted by the simulation from the curve entered in the pump database to reflect the change in water density at the pump location.  The pump operating duty point is shown at the intersection of the horizontal and vertical lines.

Pump Efficiency Curve

The efficiency curve and the flow volume intersection point define the efficiency the pump is operating at.  The efficiency value is used to calculate the estimated pump shaft power.  If pump shaft power is entered in the database directly, a pump power curve will be shown and the program will derive shaft power from this curve instead.

Pump Power Curve

If a power curve has been entered for the pump, the power curve and the flow volume intersection point will define the shaft or absorbed pump blade power the pump(s) are operating at.  Pumpsim™ will preferentially use this curve to calculate power if present.  If the power curve has not been entered for the pump, the efficiency curve will be used to calculate power.  If this is not present, the default efficiency in the settings menu will be used.

Pump NPSHr Curve

To operate without cavitation, the pump requires a minimum pressure at the entrance. If the pressure is lower than the NPSHr, Pumpsim™ will show a warning. Cavitation could make the result of the simulation less precise. The result of a model with cavitation should not be used.

Pump Database

The pump database is accessible from the TOOLS > PUMPS menu.  The pump database allows editing, adding and deleting of all pumps in the model pump database.  Up to one thousand (1000) pumps and the associated pump curves may be entered into the pump database.  A display for each pump curve and data will be presented when a pump is selected from the display list.  

The COPY and PASTE functions can be used to copy data to or from another program (for example a spreadsheet).

Conventional Pump


Fig. 3.2.5.2: The pump database. Pump information may be seen from this database; data as well as a pump curve graph is displayed.

Caution should be taken when deleting or modifying a pump, as any model using a deleted pump number will not simulate correctly.

The pump name is entered or chosen at the top from the drop-down menu.  To enter a new pump, select File > New

At a minimum, pump curve points for quantity and pump head must be entered for each pump.  Other curve information such as efficiency, power and NPSHr can also be entered.

%RPM

Adjusts the pump rotation speed from the default setting defined by the pump curve in the pump database.  Note that this is a theoretical pressure and flow adjustment which may not exactly meet true pump performance at different speeds.

Hint: While the simulated speed adjustment to a standard pump curve should be reasonably accurate, if an exact pump pressure and flow curve is required for pump performance at different revolutions, it is suggested a separate manufacturer-guaranteed pump curve be entered for the adjusted variation.

Pump Curve Density

Optional parameter, which will assume a default value if not specified.  Most manufacturers supply pump curves at a standard density, however different densities can be entered for the pump curve if required.

Pump speed

Pump speed is a manufacturers reference to the normal speed of the pump operation for the specified pump curve.  The number is not directly used during simulation and will not influence the simulation outcome.  The percentage pump speed can however be adjusted in the EDIT box.

Curve Estimation

The method used to estimate curve data between specified pump duty points.  The cubic spline method estimates a curved data path between pump points.  If only a few points of data are available, this may produce a better estimate of pump duty, however the method may be limited by sudden changes in curve data point direction.  Ensure sufficient points are available to produce a smooth non-reversing curve.

The linear method predicts a straight path between points.  This method is slightly faster during simulation, and if the maximum number of pump points (10) are entered, it should provide sufficient accuracy in most cases.

HINT:  It is important that pressure on the curve is not permitted to bend over in a U shape (or upside down U), otherwise the simulation may oscillate between two pressure points. Stall regions of pumps should be omitted for this reason.

Comments

A comment box is available to include more information about the setup or configuration of the pump. It is not used for simulation.

Point Table

The Pump Point Table will allow direct entry of pump curve data.  Pump curves will be constructed as data is entered.  Points can be submitted non-sequentially, and will be automatically rearranged when the pump is re-loaded or saved.

As a minimum, Pumpsim™ requires flow and pump head.  Other information such as efficiency or power  can be calculated by Pumpsim™ using default settings, however it is recommended to enter one of these values if available to enable more accurate calculations.

To calculate pump power within a model, Pumpsim™ needs either a pump efficiency curve or a pump power curve.  If neither of these curves is available, the default pump efficiency will be used from the Settings menu. If both efficiency and power curves are entered, Pumpsim™ will preferentially use the pump power curve to calculate absorbed pump power.

Positive Displacement Pump

Positive displacement pumps are designed to work at high pressure with little influence on the flow. The flow changes linearly depending on the RPM.


Fig. 3.2.5.3: Data for a positive displacement pump.

Pump Database Menu


Fig. 3.2.5.4: The estimate tools menu.

Estimate Tools Menu

The tools menu contains a number of functions to assist in estimating pump Total Efficiency and pump Absorbed Power if either one is unavailable.

To estimate absorbed power, Pumpsim™ will calculate theoretical power using total head and flow. Note that none of these estimation methods take into account shock losses, resistance and compression factors which will slightly affect calculated pressure and powers. It therefore should be used as a guide only, and is no substitute for an accurate pump manufactures curve if available.

Estimate Pump Curve (Interpolate Points)

Quickly estimates 10 evenly spaced points of pump curve duty from as few as 3 entered points. The estimation method will use the Cubic Spline method to add the additional points. This method should only be used if additional pump curve data is not accurately available.

Compressor Database

Compressors usually do not have a curve like pumps. To simulate them, enter the operating and maximum flow and pressure which must be available from the manufacturer. Pumpsim™ will then create a pump curve using those points.