Applies To Product(s): Bentley HAMMER Version(s): V8i Environment: N/A Area: Modeling Subarea: N/A Original Author: Jesse Dringoli, Bentley Technical Support Group Overview HAMMER provides several different ways to model a check valve, otherwise known as a “non-return” or “backflow preventer”. The following technote explains how each type of check valve works and when to use it. Pipe Check Valve The first and simplest way to model a check valve in HAMMER is to place it on a pipe element. This is done by simply selecting “true” for the “has check valve” property of the pipe. This would be done on the pipe where the check valve would be located. With this check valve approach, the following is assumed during the transient simulation: The check valve is located at the center point of the pipe The check valve closes instantly on first detection of reverse flow When using a check valve on a pipe, you will notice a symbol appear on the pipe: Pump Check Valve Another way to model a check valve is through the pump node element. If you’re modeling a Shut After Time Delay event, this is done by selecting “Check Valve” for the “Pump Valve Type”. If you’re modeling a Pump Startup or Variable Speed event, this is done by entering a zero for the “Time (For Valve To Operate)” field. This approach would be used if you have a check valve built into your pump, or if you’d like to assume the distance between the pump and downstream check valve is negligible. Similar to the pipe check valve, an instant closure is assumed, upon first detection of reverse flow. The pump node itself closes, preventing reverse flow. Check Valve Node The third way to model a check valve is by using the Check Valve node element. Simply place the check valve node in place of a junction, or even along the length of the pipe where the check valve exists (choose ‘yes’ when asked if you’d like to split the pipe). The check valve node approach should be used when: You’d like to see the check valve as a node, for visual or reporting purposes. You need to model a check valve that has a delayed closure instead of an instant closure. You need to model other advanced things, such as the pressure required to re-open the valve. The check valve node element is more flexible than the pump and pipe check valves, since it provides the following settings: Closing Time: The time to close the valve, from the fully open position, after reverse flow is sensed. This also establishes the linear rate of closure used if the valve is partially open when it starts to close. Set this to zero for instantaneous closure. Pressure Threshold –The pressure difference between the upstream and downstream side needed to reopen a closed check valve. If you select zero for this, the valve will reopen as soon as the upstream pressure exceeds the downstream pressure. Opening Time: The time to open the valve, from the fully closed position, after the specified valve opening pressure threshold is exceeded. This also establishes the linear rate of opening used if the valve is partially closed when it starts to open. Set this to zero for instantaneous opening Allow Disruption of Operation?: Denotes whether an operation (opening or closing) can be terminated prematurely due to a signal to reverse. False means that an opening or closing operation must complete once it starts (useful, for example, if the check valve is motorized and must fully close or open). True means that an opening or closing operation may be aborted at any time if the system conditions dictate. For example, if the check valve is half closed and the system pressures change (so that upstream pressure becomes higher than downstream pressure), then the valve will start to open again. Normally this field will be set to ‘True’. When using the check valve node element, you will see a user notification message for each change in the check valve position (starting to open, starting to close and interruptions) Introducing a delay in the closure of a check valve may prove to be more accurate, and sometimes more conservative. The reason is because in the case of an instantly closing check valve, the water column is essentially at rest at the time when it closes (zero flow). In contrast, if it takes some amount of time to close the check valve, momentum from the reverse flow is allowed to build up before the closure. Therefore the water column has some energy when the check valve closes, often resulting in more severe upsurge pressures. Consider the below graphs, which show the head and flow over time for an emergency pump shut down event, comparing instant closure (using either the pipe or pump check valve) to slow closure with a delay of 0.1 seconds and 0.5 seconds (using the check valve node). In the two slow closure cases, you can see that the peak HGL is higher than in the instant closure case. After 5 seconds, you can see from the flow graph that some reverse velocity has built up as the check valve closes, causing the increased pressure surge when the valve has fully closed. You can also observe that a closure delay of 0.5 seconds results in a higher pressure surge than a delay of 0.1 seconds, due to the increased time for reverse velocity to increase. See Also Reverse Velocity vs. Deceleration curves for a check valve Haestad Methods Product Tech Notes And FAQs Protective Equipment FAQ General HAMMER V8i FAQ
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