There are approximately 1,500 coal-fired power generation units and 104 nuclear reactors in the United States. Each plant’s boiler unit will have at least two boiler feedwater pumps that increase the pressure of the condensate water and discharge into the boiler. Most of these pumps are between-bearings, multistage centrifugal designs with mechanical seals installed at each end of the pump. Although the number of mechanical seals installed in these applications is unknown, it must certainly be in the thousands. Many of the pumps use shaft packing or crude support systems that are due for technology upgrades.
The sealing conditions for boiler feedwater pumps range from 20 to 200 barg [290 to 2,900 PSIG], up to 175 C (347 F), and shaft speeds between 3,600 and 5,500 RPM. For safe and reliable mechanical seal operation the water temperature must be below 65 C (150 F) where the viscosity is above 0.43 cP. At temperatures higher than this, the viscosity of the water decreases to a point where a stable fluid film cannot form between the seal faces, and seal damage occurs. Maintaining the proper sealing temperature can be accomplished in three ways:
- Inject cool external water into the seal area at or above seal chamber pressure (API Plan 32), which is extremely energy inefficient and unreliable.
- Install a bypass line from the discharge of an intermediate impeller stage of the pump, route the water through a heat exchanger, and inject back into the seal chamber at reduced temperature and pressure. This method (API Plan 21) is also very energy inefficient and quite unreliable.
- Circulate a quasi-closed loop of process water from the seal to a heat exchanger, then back to the seal (API Plan 23). This is the most energy efficient and reliable method of sealing boiler feedwater.
|Figure 1. Piping diagram for the Plan 23 system showing the major components and flow path. ?(TI = temperature indicator, TT = temperature transmitter)|
A modern Plan 23 system is shown in figures 1 and 2. One system is required for each mechanical seal, so in the case of a between-bearings pump there will be two separate systems — one for the drive end (DE) and one for the non-drive end (NDE). Five major components comprise this system.
- Mechanical seal — The seal has a circulating feature built into the mechanical seal that relies on shaft speed to create a flow of 8–40 L/min (2–10 GPM).
- Separation tank — An air-water separation tank functions as a pre-separation device for the vent valve and as an instrument vessel for temperature transmitters or other monitoring devices. One of the most important requirements of a Plan 23 system is the ability to vent air from the cooling loop so that the air does not form around the seal faces and cause the faces to run dry. That would cause an immediate seal failure. Venting must occur during the pump startup procedure and then periodically during pump operation.
- Ball float vent valve — This device is a small vessel with a float-actuated valve that opens when air accumulates at the top of the vessel. The air is released to the vent header, which is routed to the common sump.
- Filter assembly — Comprised of a magnetic filter and bypass valves, this component removes rust and pipe scale from the loop to prevent contamination of the mechanical seal. Depending on construction, the filter may require venting at startup whereupon this vent line is routed to the sump.
- Heat exchanger — The heat exchanger is a compact shell-and-tube design through which plant cooling water is used as the cooling medium. The heat loads acting on the process side of the exchanger are seal-face-generated heat and heat soak. Typical heat transfer rates can range from 5 kW to 25 kW (17,000 to 85,000 BTU/hr). These exchangers are normally manufactured by the mechanical seal vendor.
|Figure 2. The Plan 23 system installed on a multistage centrifugal boiler feedwater pump (courtesy of KSB, ksb.com).|
The described system addresses three major issues surrounding the venting procedure:
- Safety: Boiler feedwater is at high pressure. Cracking any vent valves can release a jet of water and vapor that can be a safety hazard if not addressed with engineering controls and operator training. Boiler feedwater can be at high temperatures depending on the startup and operation of the pump and seal system. Releasing a stream of hot water or steam into the plant environment can seriously burn plant personnel. Steam burns present unique risks since steam discharge can be invisible and steam burns release much more heat into skin due to the release of latent heat. Hot surfaces or work areas should be labeled in compliance with federal, state or local requirements. This is why all vent lines must be routed to a low-point sump.
- Reliability: Operating a conventional mechanical seal on air for even a few seconds can shorten the life of the seal or cause a complete failure. If the life of the seal is expected to exceed three or more years, the venting procedure must be treated very seriously. Too much venting will draw hot process water past the throat bushing and into the Plan 23 loop, causing an increase of temperature of the water at the seal and o-rings. The ball float vent valve prevents this.
- Environment: It is a poor practice to discharge any process fluids to the environment even if the fluid is water. Stray moisture can enter bearing housings, electric motors, pneumatic or solenoid controlled valves. Water can cause instrument or structural components to rust. Stray water can cause vapor clouds that reduce visibility or ice that causes a slip hazard. Again, this is why it is important to route all vents through tubing to the sump area.
For a plant that is upgrading from packing or older seal technology, the cost of a modern Plan 23 system may seem excessive. However, time and time again this solution has proven to be the lowest lifecycle cost while at the same time maximizing the safety, reliability and environmental elements. This justification burden usually falls on plant management since this requires funding and possibly extending the preventive maintenance downtime for the initial installation and testing. At many of the plants I have visited, the operators and maintenance teams have welcomed the modern Plan 23 solution, but are cynical about getting the money to implement. Maybe the Safety-Reliability-Environmental approach will help, as would a field test with actual financial data conducted at a willing plant site.
John Merrill is the U.S. application engineer for EagleBurgmann Industries, a manufacturer of mechanical seals and sealing solutions. Mr. Merrill has been designing, installing, troubleshooting, and testing mechanical face seals and seal support systems since 1991. His market sector experience includes upstream and downstream oil and gas, mining, chemical production, biopharmaceuticals, fossil and nuclear power generation, pulp and paper, and municipal wastewater. Mr. Merrill currently serves as chairman of the Hydraulic Institute’s Seals Committee and participates in the MTBR and Vibration Committees. He can be reached at email@example.com or 704-525-9672.