Modeling Reference - Valve Closure

 Applies To Product(s): HAMMER, WaterGEMS, WaterCAD Version(s): V8i, CONNECT Edition Area: Modeling Original Author: Jesse Dringoli, Bentley Technical Support Group

Overview

This technote explains how various types of valves work and their typical application in HAMMER and in WaterGEM / WaterCAD. It also provides an example model file for demonstration purposes.

Primary Valve Elements

The main "valves" icon on the layout toolbar offers several generic valves: GPV, TCV, FCV, PRV, PSV and PBV. These are sometimes collectively referred to as "valves of various types" in older versions of HAMMER.

These valve elements serve various purposes and are frequently used in steady state or Extended Period hydraulic models. They can generally be categorized as flow control. For example, the GPV defines a curve of flow versus headloss, the FCV (Flow Control Valve) controls flow to a set point and the PRV (Pressure Reducing Valve) controls the downstream pressure to be below a set point.

The important thing to understand about dynamic valves like FCVs, PSVs and PRVs is that their controlling effects (pressure reduction, flow control, etc) only apply to the initial conditions calculation (steady state or EPS). For a given steady state or EPS timestep, a specific headloss occurs across the valve. For example, in order for the PSV element to sustain upstream pressure, a specific headloss occurs through the valve, such that in order to balance energy across the network, the upstream hydraulic grade ends up being higher than the PSV set point. During the transient simulation, a discharge coefficient is calculated based on that valve's headloss during the initial conditions timestep.

`Note: the following equation is used to convert between headloss coefficient and discharge coefficient: H = 39.693 * D^4/Cv^2 Where: D = Diameter (ft), H = Headloss coefficient (K), Cv = discharge coefficient (cfs/ftH20^0.5) It can be re-written as: Cv = ((39.693 * D^4) / H)^0.5`

As the transient simulation progresses and the system conditions change, these valves will not automatically react, like they do during the initial conditions. Meaning, if the transient conditions cause a higher flow through a FCV, it will not automatically throttle (change its headloss) to react accordingly. The reason is because HAMMER assumes that these valves cannot react fast enough during the transient simulation. So, they will stay in a fixed position based on the aforementioned discharge coefficient from the initial conditions.

However, the user can manually open and close these valves during the transient simulation by using the Operating Rule. The operating rule is an attribute of the valve, found under the "Transient (Operational)" section of the properties. It allows you to define a pattern of Time versus Relative closure, to be using during the transient simulation.

Note: PRVs are currently an exception to this, as they have the ability to modulate (throttle automatically to meet a setpoint) during the transient simulation. See related article: Using Modulating PRVs

For example, if you have a gate valve that is fully open in the initial conditions and you want it to fully close during the transient simulation, you could define your operating rule with a starting relative closure of 0% and pattern that rises to 100% at some point. You would then select that in the valve's properties, under the "Operating Rule" field. You can analyze the effect of various closure patterns either by manually changing the operating rule and re-running the simulation or by creating multiple scenarios, computing a batch run in the scenario manager and then individually examining the transient results.

`Note: The operating rule designation is stored in the "Transient" alternative.  `

Generally the transient surge will be more severe for a faster closure. So, typically the last bit of closure should occur slowly. For example, the valve may close quickly between 0% and 95% closure, then slowly close for the last 5%. However, you may want to analyze the worst case scenario where the valve is closed too quickly. The speed of closure can easily be reflected in your Operating rule pattern.

`Note: Do not confuse the Operating Rule with the "Pattern (Valve Settings)" or "Pattern (relative closure)". The latter two fields may be found under the "physical" section of the valve's properties and are used to establish a manual closure pattern for the valve during the initial conditions (EPS) only.  `

For general valve closure purposes such as gate valves, isolation valves, etc, it is recommended that you use the TCV valve type (throttle control valve). This valve represents a standard headloss or discharge coefficient during the initial conditions. So, in the above example of an initially open valve, you would specify the loss coefficient representing the losses through the fully open valve.

Other Valve Types

Valve with Linear Area Change - The "Valve with Linear Area Change" element represents a simplified valve that either closes linearly (with respect to area) or acts as a check valve that stays closed upon reverse flow. The user only specifies a time to close, so no delay can be incorporated with the closure. Meaning, it starts closing as soon as the transient simulation begins. See more here: Modeling Reference - Valve With Linear Area Change

Pump valves - The pump element has a built-in valve, that can either operate as a check valve (when the "Pump Valve Type" is set to "check valve") or a linearly closing valve (with the "Pump Valve Type" set to "Control Valve"). The Control valve will either open or close over a given duration, depending on the initial status of the pump (on or off). See also: Modeling a pump that has neither a check valve nor a control valve

Check valves - a check valve can be simulated in a pipe, as a separate node element, and built into a pump. These close upon reverse flow. A slow closing operation can be modeled with the check valve node element. More on this here: Modeling Reference - Check Valves

Isolation valve - this element can be associated with the pipe, so it has the benefit of not adding an extra pipe from a split. However, the operation of this type of valve cannot be simulated during a transient simulation. Meaning, it can only be used to set the initial status of the related pipe to open or closed. If you need to control an isolation valve, use the TCV element instead.

Initially Partially Closed Valves

The TCV also has the ability to model the opening/closing of a valve that is initially partially closed. This is done by way of the "valve characteristics curve" coefficient type and "Valve Type". Normally, the discharge coefficient that the program computes based on what the valve is doing during the initial conditions is interpretted by HAMMER as the fully open position. For example, if you use a GPV, the headloss calculated through it in the initial conditions is always interpretted as a relative closure of 0%, even though in reality, it may be partially closed. This can cause confusion when defining the operating rule. However, with the valve characteristics curve coefficient type for a TCV, the user can define the relationship of discharge coefficient versus relative closure; therefore, a partially closed valve can be properly modeled. Detailed instructions on how to do this are beyond the scope of this technote. For more information, please see Modeling An Initially Partially Closed Valve

Custom Valve Characteristics

By default, several standard valve types are available, in the "valve type" field (such as butterfly, globe, needle). This essentially defines the discharge coefficient that HAMMER uses for various values of "relative closure" in the operating rule. For example, some valves may have a sharp reduction in area as they start to close (stroke) and then a slower reduction in area just before they are fully closed. Meaning, a value of 90% closure in your operating rule might not necessarily mean the valve's open area is 10% of the original area. The user can also define a user defined table of relative closure versus relative discharge coefficient, by selecting "user defined" as the valve type.

This exposes the "Valve Characteristics" attribute, which is where you would enter the table of relative closure versus relative discharge coefficient to define the characteristics of your valve. The relative discharge coefficient values are relative to the value entered for "Discharge Coefficient (fully open)".

Valve Characteristic Curves

Valve characterisic cuves for standard types are based on published data (Fok, 1987). The curves have the functional form:

1 – Yk ...

where needle valves have k = 2.0; circular gate valves, k = 1.35; and globe valves k = 1.0;

or

(1 – Y )k ...

where for ball valves, k = 1.35; and butterfly valves, k = 1.85.

Fok, A.T.K., “A Contribution to the Analysis of Energy Losses in Transient Pipe Flow”, Ph.D. Thesis, University of Ottawa, 1987.

`Note: most valve manufacturers can provide the discharge coefficient(s)  `

`Note: When modeling a valve whose initial status is "inactive", ensure that you've entered a value for the "minor loss coefficient". When computing initial conditions, the "minor loss coefficient" is used to compute headloss through the fully open (inactive) valve. This headloss is important since it is used to define the relationship between head loss and discharge as the valve closes. `

Example Model

In the latest version of HAMMER, this example model can be found in the "Samples" folder within the installation folder and is called "Valve_Closure_Example.wtg". For older versions that do not have this, you can download a version of it here:

Note: the above model is for example purposes only. It can be opened in version 08.11.00.30 and above and you can find additional information under File > Project Properties. Also, you must be signed in to Bentley Communities or the link will not work.

Video Demonstration - transient valve closure

A video demonstration of advanced valve closure modeling is available within the HAMMER Learning path on Learn.bentley.com. See the on-demand video named "Causes of Transients - Valves"

Reference

1. Advanced Water Distribution Modeling and Management - Walski, 2007

Gradual closure not occurring when using valve operating rule

Protective Equipment FAQ

Modeling An Initially Partially Closed Valve

Modeling Reference - Valve With Linear Area Change

Using Modulating PRVs

General HAMMER V8i FAQ

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