Specific Speed and Pump Flow Calculations for a Transient Simulation

Applies To
Product(s): HAMMER
Version(s): CONNECT Edition, V8i
Area: Modeling
Original Author: Scott Kampa, Bentley Technical Support Group

Problem Description

How is pump flow calculated during a transient simulation in HAMMER?

What exactly is the Specific Speed seen in the transient tab of the pump definition, and how is it used?

Background

There are some additional points of interest with pump flow in the transient analysis compared to the pump operation in the initial conditions or in WaterGEMS/WaterCAD.

In a steady state or EPS in WaterGEMS and WaterCAD (and in the initial conditions calculation for HAMMER), reverse flow through the pump is not allowed, and the pump will only operate in a single quadrant: positive head, flow and speed. In HAMMER, you can have cases where reverse flow through a pump is possible (and reverse speed of the impeller). Because of this, a four-quadrant curve is used to define the characteristics in the transient analysis, and therefore influence the pump flow calculations.

HAMMER includes several pre-defined four-quadrant curves (source) by way of the Specific Speed selection. These are selected by going to the transient tab of the pump definition dialog, found by going to Components > Pump Definition.

The specific speed provides four-quadrant characteristic curves to represent typical pumps for each of the most common types.

The specific speed is a function of the impeller geometry and streamlines, and essentially defines the shape of the four-quadrant representation of the pump. HAMMER needs this additional information because of the possibility of reverse flow and speed during a transient simulation. Each Specific Speed corresponds to an internal curve whose values are a percentage of the "rated" flow, head and speed. The "rated" conditions are from the initial conditions calculation - the pump's initial head, flow and speed. So, it is important that the pump be operating at or near its best efficiency point during the initial conditions, so as not to skew the four-quadrant representation.

Estimating Specific Speed for your Pump

The specific speed you select should be as close as possible to the manufacturer's catalog to assure the correct values. See below article on selecting an appropriate specific speed: Estimating the Specific Speed of a Pump or Turbine

While the specific speed defines the four-quadrant curve that is used, the curve itself is dimensionless. Because of this, the initial pump operating point (the head(nominal) and flow(nominal) from the initial conditions) is still important. All of the points on the four-quadrant curve are relative to initial operating point. The quadrant curve data is dimensionless, so a change to the initial conditions operating point of the pump can have an effect on the quadrant curve that is used. Additionally, HAMMER assumes that the pump or turbine is operating at the best efficiency point.

Transient Pump Operating Point vs. Pump Definition

Because HAMMER can use four-quadrant curves, you will sometimes see a case where the pumps seem to flow at values that do not match up with the pump definition. When this occurs, you will likely need to look into the pump definition and specific speed used to assure that the setup of the pump is correct and matches well with the expected pump flow.

An extreme example would be a pump that starts operating near the best efficiency point in the initial conditions, but then a downstream valve closes and the pump is pumping against a closed valve. You might expect the shutoff head point to match the pump definition at this point during the transient simulation, but the head value may differ, because it is operating on the four-quadrant curve, at a point relatively far from the initial operating point. In such a case, if the pump is not changing speed during the transient simulation, set the transient pump type to "constant speed - pump curve", which will force HAMMER to only stay in the first quadrant, following the user-entered pump definition. If this is a pump that changes speed (shuts down/turns on), then you may need to accept this difference in shutoff head, or try a different specific speed.

In addition to the above, the nominal (full) speed and inertia (also assigned in the Transient tab of the pump definition) can impact the results. Changes to speed and inertia will affect hydraulics. This then could cause a change in where the pump operates. For example, the inertia affects how the pump changes speed, and the speed is part of the four-quadrant curve. The pump definition that you entered for your pump represents 100% (full) speed, so a change in speed during the transient simulation will cause the pump characteristics to shift up and down, as another factor to consider. In particular, this can happen when using the "shut after time delay" transient pump type, if hydraulic conditions change before the time that the pump shuts down (again, the example of a valve closing before the pump shutdown). See: Pump speed not constant before emergency pump shutdown transient event

Taking into account that head and flow are used as well, you can see that everything is interrelated, and the proper input is needed to assure accurate results.

Custom Specific Speed (Four Quadrant Curve)

Note that you can also construct your own four-quadrant curve, which could give you more control over the results by better matching your exact pump. However, this data can often be difficult to obtain and difficult to convert into the required format, so it is still recommended to try one of the default Specific Speeds closest to your pump based on the equation in the above article. You can find more information on making your own four-quadrant curve at the Help topic "Pump and Turbine Characteristics in Bentley HAMMER." and in this article: Defining custom four-quadrant curves (specific speed) for pumps and turbines in HAMMER

See More

[Help topic] Pump and Turbine Characteristics in HAMMER 

Recommended
Related