Transient simulation for a hydrant closure or opening

Product(s): HAMMER
Version(s): V8i, CONNECT Edition
Area: Modeling

Problem

How can I model a transient simulation of a hydrant closing or opening too quickly?

Solution

A hydrant closure or opening transient event can be modeled in HAMMER using one of two approaches:

Discharge To Atmosphere

A Discharge to Atmosphere (D2A) element can be used to model a hydrant closure, if you are comfortable with assuming a simple linear closure. First, lay out a D2A node element at the hydrant location. Set "discharge element type" to "Valve", and the "Valve Initial Status" appropriately. If you're modeling a hydrant closing, set it to "open". If you're modeling a hydrant opening, set it to "closed".

The characteristics of the opening are modeled by way of the "typical flow" and "typical pressure drop". See more on this in the relevant section of the article linked to at the bottom of this article.

For a hydrant, you might enter the typical pressure and flow as the flow and pressure observed in a field test when the hydrant was opened. You are basically defining an orifice size by way of the "typical" flow and pressure drop fields. By supplying one pair of pressure and flow, HAMMER can figure out the relationship based on the orifice equation that gives the pressure drop for any flow value. So, if unsure, you can use the orifice equation along with the size of your opening and an estimate of the "head" (pressure head drop) to solve for the typical flow. Selecting a pressure head drop close to a typical value you might see under normal operating conditions will yield the most accurate pressure/flow relationship during both the initial conditions and transient simulation. 

Note: a standard 2.5 in. (100 mm) hydrant outlet would have a pressure drop of roughly 10 psi at 500 gpm. 

For an example of this, see the hydrant scenarios in the example model "Discharge_To_Atmosphere_Example.wtg", included in your HAMMER installation. It is located in the "Samples" folder within the installation folder.

Valve

If you need more control over the closure of the valve itself, you can use the Throttle Control Valve (TCV) element, with a pipe downstream of it followed by a demand or a D2A. The demand would be set up to model the hydrant flow. If the hydrant should be initially closed, use a D2A node downstream of the valve instead, as its outflow is pressure dependent even in the initial conditions and therefore it will have zero outflow when the valve is initially closed, yet will have outflow when it starts to open during the transient simulation.

If using a D2A downstream of the TCV, configure its "Discharge element type" as "orifice" and enter the typical flow and pressure drop fields (see above and in the link below for more information on that).

For the TCV, you can then configure the coefficient type, initial position, valve characteristics, and operating rule pattern. This gives you full control over what the hydrant is doing including how fast it is opened or closed. You can even model a closure and opening in the same simulation using an appropriate operating rule. See more in the link below for modeling the TCV.

See Also

Modeling Reference - Discharge To Atmosphere

Modeling Reference - Valves

Modeling An Initially Partially Closed Valve

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