PIPE-FLO controls provide you with the ability to control your system to meet its operating requirements. Often, too many controls are placed in the system in an attempt to achieve the desired operating parameters. Inserting controls without knowing their effect on the total system can make the system unstable or inoperable. When placing controls in a piping system, it is mandatory that you know how the system should operate prior to installing the first control. This example, "Sample Over Controlled System," was created to demonstrate points in troubleshooting, but it is typical of systems that are sent in to our technical support staff. This knowledge base article describes how to troubleshoot piping systems with controls. It applies to existing piping systems as well as newly designed systems.
The Problem: Step 1
Download and open the .pipe file attached below . After performing the calculation, you will notice a number of the pipelines have fluid flow reversal. This is an indication that we may have a controls problem.
In looking at the controls in the system, notice the "RT-2 Control" is a Pressure Regulating Valve (PRV) set to 80 psi. This PRV is trying to maintain the downstream pressure to 80 psig. For this to occur, PIPE-FLO calculated that the flow rate through the pipeline to "RT-2" had to be 1194 gpm. This is the flow rate that would increase the PRV's downstream pressure to 80 psig.
Since the pump "RP-1" is set to run at 200 gpm, and the flow to tank "RS-1" is set to 80 gpm by the "RS-1 Control," the remaining flow needs to come from tank "RS-2." Hence, the cause for the reversal of flow. The only way you can get 1194 gpm to flow through the pipeline to "RT-2" is to have a differential pressure "gain" of 297.2 psi across the "RT-2 Control" (looking at the differential pressure needed.) This would require that the inlet pressure of the "RT-2 control" be -217.2 psig. This value is well below absolute zero pressure and as a result, this condition is impossible to attain. (Another indication the system has a controls problem.) PIPE-FLO gives warning messages that the pressure is less than absolute zero.
In talking with the customer, he mentioned that the control valve "RT-2 Control" was installed so the system pressure does not exceed 80 psi. If the pressure exceeds 80 psi the valve opens and relieves pressure to tank RT-2, essentially acting as a BPV at 80psi when open. For now, instead of replacing the PRV with a BPV, I will be closing the "RT-2 Control" and re-running the analysis.
The Solution: Step 1
The first step to troubleshooting the system is to close the "RT-2 Control" and re-run the calculation.
I created a lineup called "1. Close RT-2 Control". Switching to that lineup and performing the calculation eliminates the reversal of flow in the pipelines, but reveals another set of problems.
The Problem: Step 2
The customer wants a flow of 80 gpm to tank "RS-1", that is why the "RS-1 Control" is an FCV set to 80 gpm. They also want a flow of 120 gpm into tank "RS-2". The remaining 20 gpm is for a constant minimum flow through the pump when flow is stopped to tanks "RS-1" and "RS-2".
Since he had not selected a pump, I asked why he chose 20 gpm as the minimum flow. He said that was approximately 10% flow.
In looking at the results, notice there is around 118 gpm going through the "Minimum Flow Line." Also notice that for the pipeline called "To RS-2," that there is only ~2 gpm as opposed to the desired 120 gpm. Finally, notice that the FCV called "RS-1 control" is unable to meet the desired flow rate. That is because there is insufficient pump head to pass 80 gpm into tank "RS-1."
The Solution: Step 2
To correct this problem we will:
- Close the minimum flow line (we will determine the actual required minimum flow for the pump once it is selected.)
- Perform a pump selection and set the pump to run on a curve.
- Re-open the minimum flow line.
A lineup called "2. Pump Sizing" shows how the system operates with the minimum line closed and the flow control operating properly. Reviewing the results, notice the differential pressure across the FCV "RS-1 Control" is only .373 psi and the RP-1 pump needs to develop a minimum of 72.78 feet of head. This is the information required to size a pump for the system.
The remainder of work on this system involves:
- Selecting a pump and placing it into the system to run on a curve
- Placing a flow control valve called "RS-2 Control" into the system to regulate the flow into tank RS-2 to 120 gpm
- Selecting the "RS-1 Control" and "RS-2 control" flow control valves
- Placing the control valves in the system to see how everything operates
- Sizing the balancing orifice needed to limit the flow rate in the minimum flow line to the required value (based on the manufacturer's requirement for the pump.)
- Determining the maximum system pressure with the pump installed, then sizing the over pressurization line accordingly.
NOTE: The files with the ".pipe" file extension can only be opened with version 8 and above.
If you do not have PIPE-FLO version 8 or above, you can open the ".psv" file using the PIPE-FLO Viewer program that can be downloaded from the PIPE-FLO_Viewer_Download_Page.