Bulletin #:005_07

Date:03-26-2007

Category:1

Subject:Choked control valve condition not identified when set at an Automatic Set Value

Program affected:PIPE-FLO Professional, Flow of Fluids

Approved by:RTH

Background

Choked flow is a limiting, or maximum, flow rate achievable through a control valve. This maximum flow rate is defined as QMax. When the fluid zone is liquid, choking occurs as a result of vaporization of the liquid when the pressure within the valve falls below the vapor pressure of the liquid. This occurs as either cavitation if the downstream pressure is greater than the vapor pressure of the liquid, or flashing if the vaporization only occurs at the vena contracta of the control valve. When the fluid zone is a vapor, choking occurs due to sonic velocity at the vena contracta point.

A control valve (Flow Control, Pressure Regulating, or Back Pressure Valve) may be entered in PIPE-FLO with or without valve data. If entered without valve data, the program returns a dP across the valve and the necessary flow rate to maintain the valve's design condition. This allows you to size & select a control valve based on system conditions. During your valve selection, care should be taken to select a valve with a Cv value at your design conditions such that choked flow does not occur.

If the control valve is entered with Cv and Fl (or Xt) data, the program displays flow rate, % Open, and dP across the valve. If the valve is set to Fully Open or a Manual Set Position, the program also calculates QMax, checks for choked flow and, if choked, will flag the valve red. When choking occurs in a valve, a disconnect in the system occurs since the dP through the control valve no longer controls the flow. This condition invalidates PIPE-FLO's calculated results.

Condition

If the valve is defined with Cv and Fl (or Xt) data, and is set as an Automatic Set Value, the program does not calculate QMax and does not check for choked flow.

Workaround

To force the program to check for choked flow:

1) Select the control in question and read it's Status to find the % open

2) Double click the control and set it to "Manual Set Position" at the value found in step 1.

3) Calculate results. If your status is Choked, the valve that is selected will produce a choked flow condition and your results are not valid.

To find a valve that will operate without choking:

1) Delete the current valve data by deleting the valve and creating a new one on the node and calculate the dP required across the valve.

2) Use this information in discussions with your valve manufacturer.