Ray Hardee is a principal founder of Engineered Software, creators of PIPE-FLO and PUMP-FLO software. At Engineered Software, he helped develop two training courses and teaches these courses internationally. He may be reached at email@example.com.
If there is erosion or wear between the valve disk and valve seat, the control valve needs to be closed more than expected to meet the flow conditions. After checking with the maintenance department, personnel discovered no work had been done recently on the level control valve. After checking with operations, the workers found the valve operated normally before the system had problems meeting the process requirements.
The other potential cause for the control valve being closed more than normal would be a change in the system's static head. As indicated in Table 1, the tank levels were normal, and the pressure in the process tank was 22 psi, a value that did not vary. An operator was sent to the tank to check the local level on the sight glass and the pressure gauge attached to the process tank. The pressure gauge indicated 10 psi, 12 psi less than observed on the control panel.
Discussions with the instrument department revealed that the process vessel's pressure transmitter connected to the plant's distributed control system was scheduled to be calibrated within the month. A follow-up discussion revealed the inoperative or failed position of the pressure transmitter occurred at 22 pounds per square inch gauge (psig).
Based on the local operation and the failed position of the pressure transmitter, the indicated tank pressure was actually 10 psig instead of 22 psig. Over time, the process tank pressure decreased to 10 psi as indicated on the pressure gauge, but because the pressure transmitter was inoperative, the control loop showed 22 psi.
Once the pressure transmitter was recalibrated, the tank pressure increased to the 20 psi set value, and the system provided sufficient flow to the process loads to meet its transient flow conditions.