Control valves

Control valves are subjected to a lot of mechanical issues like wear and tear, deterioration with time. Measurement devices these days are very robust and most would not deteriorate drastically with time.  The running controllers are even more reliable.  Therefore the weakest link in the process control loop is frequently the final control element.

There are general rules and procedures associated with disassembling and reassembling control valves. and never ignore the manufacturer's installation, operation, and maintenance (IOM) instructions for every each valve model. .If the valve must be disassembled to inspect or replacement of leaking gaskets, the process line must be depressurized, or drained. Sometimes decontamination is needed if the fluids are dangerous to human.

The problems of control valves usually manifest themselves as stiction, deadband and hysteresis, which cannot be solved by controller setting ( PID mostly ). Off site the valve can undergo a full service and check, which includes bead blasting in house. This means a careful cleaning process.Then a thorough inspection of all parts takes place. The castings are depth checked for wall thickness and corrosion. Plugs and seats are inspected for erosion and the nuts and studs are replaced.

Sometimes machining of replacement parts is necessary. If a control valve is being serviced all soft parts (packing, gaskets, etc) will be replaced.