Modeling Reference - Surge Valves

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

Overview

This technote explains how the Surge Valve element works and its typical application in HAMMER V8i. It also provides an example model file for demonstration purposes. 

 

How it Works

The Surge Valve element encompasses both a Surge Anticipator Valve (SAV) and Surge Relief Valve (SRV). You can configure the valve to act as one of these types or both.  

The main purpose of the surge valve is to alleviate high pressure transients (upsurges) that can occur in some situations. This is done by opening at key times during the transient simulation. 

Note: it is important to understand that when this valve opens, it discharges to atmosphere, not into a connected pipe element. Also, if subatmospheric pressure occurs at the surge valve location, air inflow is not modeled. However, if subatmospheric pressure is simulated, it is likely that other surge protection approaches should be considered. 

 

 

 

Usage - Surge Anticipator Valve

When the pressure at the SAV valve falls below the threshold value, it opens up to the atmosphere in anticipation of a subsequent upsurge (high pressure.) This typically happens in the event of an emergency pump shut down, so the valve is usually installed at the discharge header. The anticipator valve is already open when the high-pressure transient reaches the valve; sensing of high pressure is not required to initiate the opening of the valve. The valve then closes slowly so as not to cause a transient. In some cases, these valves can be a cheap protection alternative to a surge tank or gas vessel (hydropneumatic tank), but are typically not as effective. This type of valve is more effective than a SRV when high-pressure transients occur so quickly that the SRV does not have adequate time to respond and open. Care must be taken in setting the low pressure activation point to avoid premature opening before the pump has spun down, since that can cause a very steep negative transient wave.

 

 

Note: The SAV is treated as a junction with no demand during the initial conditions (simulating the closed condition) 

Note: air inflow is not supported if the pressure drops below zero.

 

Attributes - Surge Anticipator Valve 

"Threshold Pressure (SAV)" - This is the pressure below which the valve opens up to the atmosphere. 

"Discharge Coefficient (when SAV fully open)" - This is located under the "Transient (physical)" section of the properties and is used to define the discharge coefficient (Cv) of the SAV when it is fully open. Basically at the time when the valve is fully open, the discharge out of it (to the atmosphere) is represented by this coefficient. 

"Diameter" - This is not used in the HAMMER calculations but useful for display purposes. Flow through the valve is determined based on the discharge coefficient at Full Opening and valve type. It is assumed that the percent of open-area curve for each valve type corresponds to its discharge coefficient curve. 

"SAV Closure Trigger" - This identifies the method used to trigger closure of the SAV, after it has opened. It can either be based on time or a threshold pressure. When time based, the user must enter a specific duration that the valve is open. When pressure threshold based, the SAV will begin to close when the pressure at the SAV rises back above the "threshold pressure".  

 

When using Time as the closure trigger: 

"Time for SAV to open" - Once the pressure falls below the threshold pressure, this is the time it takes for the SAV to become fully open. 

"Time SAV stays fully open" - This is the amount of time that the SAV remains fully open (the time between the end of opening phase and the start of the closing phase.) 

"Time for SAV to close" - This is the time it takes for the valve to close, measured from the time that it was completely open. 

 

When using Threshold pressure as the closure trigger: 

"Time for SAV to open" - Once the pressure falls below the threshold pressure, this is the time it takes for the SAV to become fully open. If the pressure rises back above the threshold before this time has elapsed, the valve will begin to close. If it does not rise above the threshold until after this time has elapsed, the SAV will remain fully open until that time.  

"Time for SAV to close" - This is the time it takes for the valve to close, beginning when the pressure rises above the threshold pressure. 

 

Usage - Surge Relief Valve

The SRV opens up to the atmosphere when the pressure at the SRV rises above the threshold pressure and closes immediately after pressure drops below this setting. This valve is usually installed across the pumps and discharge headers or at critical points along the pipeline, in order to control the maximum system pressure. If the SRV closes too quickly, severe transients could occur. The SRV may be too slow to open after a power failure (fast, sharp transient pulse), so it is better for protection against gradual over-pressurization. (to limit the pressure rise during normal pumping operations.) In some cases, these valves can be a cheap protection alternative to a surge tank or gas vessel (hydropneumatic tank), but may not be as effective. 

In HAMMER, the SRV is modeled as being comprised of a vertical-lift plate which is resisted by a compressed spring. At the threshold pressure, there is an equilibrium between the compressive force exerted by the valve's spring on the movable plate and the counter force applied by the pressure of the liquid. To see how the SRV outflow is calculated based on the system pressure, SRV diameter, threshold pressure and spring constant, please see the attached spreadsheet (NOTE: you will need to be logged in first or the link will not work)

Note: The SRV is treated as a junction with no demand during the initial conditions (simulating the closed condition) 

 

 

Attributes - Surge Relief Valve 

"Threshold Pressure (SRV)" - The pressure above which the SRV opens up to the atmosphere. This may be controlled by a spring, piloting, or other mechanism. 

"Diameter (SRV)" - The diameter of the opening available to release fluid from the system. It correspond to the "area" of the plate shown in the above diagram.

"Spring Constant" - Change in restoring force of the return spring per unit lift off the valve seat. A possible value is 150lb/in. For a linear spring, the lift x is given by the equation: A (P - P0) = k x, where A is the pipe area, P is the instantaneous pressure, P0 is the threshold pressure, and k is the spring constant. In this formulation, the acceleration of the spring and plate system is ignored. As the plate lifts away from the pipe due to the excess pressure, more flow can be vented to atmosphere. Currently there is no maximum imposed on how far the plate can lift.

 

Reporting

After computing the transient simulation, you will see a user notification (Analysis > User Notifications) indicating the volume of water that discharged out of your surge valve. For example: 

"The total discharge through surge anticipator valve and/or surge relief valve = 2.619 m³." 

If your surge valve is at a "Tee" (separate short pipe going from the main line to the surge valve at a dead end) then you can graph the flow over time by adding the pipe end as a report point. To do this, go to Analysis > Calculation Options, open your transient calculation options and make sure "report points" is set to "all points", or "selected points", with the surge valve added as a report point in the report point collection. Then, after computing the transient simulation, go to Analysis > Transient Results Viewer and plot a time history of "flow" for the pipe endpoint adjacent to the surge valve. 

If your surge valve is not at a "Tee" and is instead in-line / in-series with the main pipeline (two adjacent pipes), you will not be able to directly report the discharge hydrograph (in the current version of HAMMER.) You would need to plot a time history of flow for the adjacent upstream and downstream pipe endpoints and find the difference. 

 

Example Model

Click to Download

Note: you will need to be signed in or the link above will not work. 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.

 

Reference

  1. HAMMER V8i, Transient Analysis and Design training course manual (TRN013190-1/0001)

See Also

Product TechNotes and FAQs

OpenFlows Product Tech Notes And FAQs

Protective Equipment FAQ

General HAMMER V8i FAQ

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