Check Valve Dynamic Curve or Reverse Velocity vs. Deceleration Curves for a Check Valve in HAMMER

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


Problem 


How can you input Reverse Velocity vs. Deceleration curves from the check valve manufacturer into the Check Valve element in HAMMER? 

Background

The reverse velocity vs. deceleration curves from the valve manufacturer do not typically tell you much about the relationship between flow and headloss in the check valve or the rate the valve is closing. This will provide engineers with a flow velocity 'delta' to use in Joukowsky's Equation: dH = dV . a / g (where 'dH' is change in head, 'dV' is change in velocity, 'a' is wave speed and 'g' is gravitational constant - more information can be found in Section 13.3 of see Bentley's Advanced Water Distribution Modeling & Management book).

With this, you can do a quick check of the change in head in the pipe as the check valve slams shut. These curves are more of a rule of thumb than an actual physical valve parameters and should be used with appropriate caution. But they are usually the best information available from manufacturers on dynamic behavior of check valves.

By default, the check valve node and check valve pipe attribute close instantly upon sensing reverse flow (i.e. reverse velocity = 0.) If the reverse velocity is not high, then it should be ok to model them as regular check valves that close instantly.

Solution

Starting with HAMMER CONNECT Edition Update 3 (build 10.03.03.72), it is possible to apply Check Valve Dynamic Curves (or reverse velocity vs deceleration curve) to a model. In the transient simulation, reverse velocity and deceleration at a time step can be calculated. The calculated values can be compared with Check Valve Dynamic Curve to determine when the check valve will close instantaneously.

To use this feature, set the Check Valve Closure Type to “Dynamic Characteristics Curve.”

You can then select an item available in the Check Valve Dynamic Curve manager (Components > More > Check Valve Dynamic Curves). This manager will include the reverse velocity vs deceleration curve data for check valve used in the model.

During the transient simulation, reverse flow velocity and deceleration are calculated, then compared with reverse velocity and deceleration data in the check valve dynamic characteristics curve. If the calculated reverse velocity exceeds the reverse velocity in the curve at the calculated deceleration, the check valve will close instantaneously. 

With older versions, you can set the check valve element property field Check Valve Closure Type to "Slow Closing." With this setting, you can specify a check valve closure time and opening time (over which the check valve is assumed to close linearly). You can set different closing times, run the model, then review the results and look at the flow through the valve as it shuts. You might need to iterate a couple of times until the reverse flow corresponds to your reverse velocity for the valve.

FAQ's

Closing Time vs. Deceleration vs Reverse Velocity

System deceleration causes high reverse velocities, not the closing time. For higher system decelerations, the closure time is fast because the reversing water helps push the valve closed. Because deceleration is so quick, the reverse velocity is still high (even with a short closing time) and therefore likely to cause surge issues.

For more discussion on this, see the first link in the "See Also" section below.

Flow Reversal

In some cases you might not see reverse velocity or the reverse velocity reported at check valve might appear to not conform to the values characteristic curve provided. In such cases it is always a good idea to check the check valve setup. See the following troubleshooting options;

1. Ensure that the valve type for a pump is set to a Control Valve, if you are simulating a check valve after the pump. The control valve should also be set for a large delay (like 99999 seconds) if you wish to model the check valve separately and not inbuilt in the pump.

2. Ensure that the deceleration and reverse velocity values provided in the characteristic curve are "non-zero" values. Either remove the zero values or change them to very small values (say 0.0001).

3. If you wish to see the maximum reverse velocity that can be attained, choose "False" for the "Allow Disruption of Operation?". When this is set to “True” the check valve operation will be aborted if reverse flow is detected. When set to "False", the check valve operation will continue till the end of the simulation.

How does HAMMER calculate reverse velocity?

- HAMMER deceleration calculation starts when pump is shut down and flow velocity decreases. When deceleration starts(velocity start decreasing), HAMMER engine will calculate average deceleration for all time steps. When flow direction reverses, HAMMER engine will compare the reverse velocity with the reverse velocity interpolated from check valve dynamic curve using average deceleration. If the reverse velocity is equal to or larger than the reverse velocity from check valve dynamic curve, check valve will be closed.

- The deceleration curve will start tracking the values provided when the flow starts dropping but not use them immediately. However, once the values get negative, it will consider the average deceleration velocity by calculating the average of the point where flow starts dropping and the point when the flow starts accelerating (not decelerating anymore). Once the flow starts accelerating the check valve will close. However, during deceleration the program will compare the values from the curve and interpolate.

See Also

Finding Water Deceleration in Pipeline - Check Valve Slam Analysis

Modeling Reference - Check Valves in HAMMER

How can I find water deceleration in a pipeline with a check valve?

Recommended
Related