Minimizing Maintenance Costs with PIPE-FLO® Professional
The control valve in the process system at a paper mill shown in Figure 11-2 needed to be overhauled every 18 to 24 months.
In looking at the normal operating conditions of the system, the valve position was between 55 and 60% open. The operators stated that the valve had always operated with the symptoms of cavitation so they assumed the valve was designed to cavitate. The mechanics mentioned that the valve experienced cavitation damage, along with excessive wear on the valve disk. Based on the comments from the operators and maintenance staff, the engineers decided to approach their energy conservation project from a maintenance reduction effort.
The control valve in question was a 4-inch butterfly type valve located in a 6-inch diameter pipeline. The differential pressure across the control valve ranged between 35 and 40 psi. After performing a control valve capacity calculation per the ISA-S75.01 Standard Flow Equations for Sizing Control Valves, it was determined that the valve was indeed operating under cavitation conditions. This was caused by high differential pressure across the control valve, the flow rate through the valve, and relatively low downstream pressure of 5 psig. The cavitation, turbulence, vibration, and noise caused the control valve to fail prematurely.
The control valve representative suggested replacing the valve with one that could better withstand the high differential pressure occurring in this application.
This system was originally designed to pass 800 gpm and the lower flow rate was supposed to be temporary. The differential pressure across the control valve at the design flow rate of 800 gpm was specified at 10 psid in the operating system. After the pump was selected and the system was built, the differential pressure was actually close to 13 psid with the valve controlling at 800 gpm. Both of these differential pressure values are well within the capability of the selected butterfly control valve.
The problem with the valve that was originally selected was that the mill did not state that the system would be running at a reduced flow rate and higher differential pressure for a prolonged period. Based on these findings, two options were considered. Option 1 involved trimming the impeller on the Feed Pump to reduce the differential pressure across the control valve. Option 2 involved replacing the 4-inch butterfly valve with a globe valve that could better handle the higher differential pressure across the flow control valve at the reduced flow rates.
With the piping system operating at 600 gpm, the differential pressure across the control valve was between 35 and 40 psid. Based on the needs of the system and to ensure proper operation of the control valve, the engineers considered trimming the impeller on the feed pump to achieve a 10 psi pressure drop across the control valve at 600 gpm, which was well within the controlling range of the valve. Trimming the pump impeller to 11.375 inches reduces the pump head at 600 gpm to 107 feet. The resulting differential pressure across the control valve would be 10.9 psid. Based on the local labor rate, it was assumed that the changes to the pump could be made for about $2,000.
The other option required purchasing a new control valve, specifically a globe valve with cage trim. This valve design allowed for a higher differential pressure with less noise and vibration, and was less susceptible to cavitation at high differential pressures. The cost to purchase and install the new control valve was estimated at $3,500.
Option 1 was selected because all of the work could be done by the maintenance department and did not require the purchase of any new capital equipment. In addition, it was the least expensive option with the quickest simple payback period. Trimming the impeller on the Feed Pump to 11.375 inches not only reduced the differential pressure across the control valve, but also met the objectives of the energy reduction effort to use less power.
With the pump operating with the smaller impeller for 8,500 hours per year at a flow rate of 600 gpm, pump efficiency of 65%, motor efficiency of 93.5%, and a power cost of $0.10 per kWh, the pump consumed 167,056 kWh at a cost of $16,705. This created a savings of $9,209 for this pump in operating cost.
When the maintenance manager realized that trimming the impeller on the Feed Pump would eliminate one of his major problems, he had no objection to allocating his resources and scheduling the work to get the job done.