Air-operated piston assembly, injectable and air flush enhance this sealing system's performance in harsh applications
A leak prevention system (LPS) is available that is effective in sealing pumpage with particulates and for clear liquids. It can be used with pumps, mixers, agitators, ribbon blenders, fans and sealing shafts for dry powder handling and includes fast and slow rotating equipment.
The LPS is analogous to a double mechanical seal that uses a flexible sealant as a barrier fluid. This barrier fluid is maintained at a pressure slightly above stuffing box pressure by an air-operated piston. Close-fitting bushings at either end of the seal retain the flexible sealant. In essence, the LPS traps the sealant within the LPS assembly, and the pumpage has no chance to escape from the pump since it sees no leakage path.
The pressurizing sealant does not rotate with the shaft and, in remaining static, is actually a flexible stationary seat. The vertical edges of the machined crenellations (perpendicular to the shaft axis) act as rotating heads. In an LPS with two crenellations, four sealing surfaces are on the shaft sleeve.
Use with High and Low Shaft Speeds
The initial versions of the LPS were designed for low shaft speeds, with shaft peripheral velocity less than 0.5 meters/second (1.64 feet/second). These designs, typically used on blenders, mixers or progressing cavity pumps, require no cooling. Higher shaft peripheral speeds, such as those of most centrifugal pumps, require cooling of the LPS assembly. An illustration of the cooled LPS assembly as used in centrifugal pumps is shown in Figure 1.
Figure 1. Leak prevention system general arrangement
The cooling chamber is formed between the LPS housing and the bore of the existing pump stuffing box with an O-ring seal—the cooling medium does not contact the process liquid. Guide vanes in the cooling chamber distribute the flow from the in/out ports in a serpentine path.
The Kevlar bushings provide a tight clearance against the shaft sleeve, limiting any intrusion of the flexible sealant into the pump. Similarly, the graphite and Teflon bushing combination limit the flexible sealant from exiting the pump.
Crenellations on the shaft sleeve provide radial sealing surfaces to accommodate shaft movement due to deflection and/or vibration. For process pumps (e.g. ANSI or paper stock), a hook-type shaft sleeve is supplied to protect the pump shaft and to help position the impeller on the shaft as well as enabling axial travel for impeller clearance adjustment.
Since no process fluids migrate through the stuffing box, no solids will enter the seal assembly. Solids ejection vanes are furnished on the shaft sleeve to keep solids from settling on the bottom of the stuffing box.
Standard material of the seal assembly is 304SS and other materials can be furnished for compatibility with the pumped liquid.
Shaft tangential or peripheral velocity is an important consideration when selecting the shaft seal type. Calculating this parameter can be accomplished with these equations:
In the LPS, the pressure on the flexible sealant within the assembly is maintained at about 1 bar (approximately 15 psig) above stuffing box pressure. This is accomplished by an air-operated piston assembly. See Figure 2.
Figure 2. Sealant pressurizing assembly
The pressurizing piston provides a consistent and moderate pressure on the flexible sealant. This pressure is between four times and seven times the pressure in the air chamber, depending on the pump application and pressurizing device furnished. The required air pressure is calculated on a case-by-case basis.
Virtually no air consumption occurs for this device. If air pressure is lost, the seal will continue to operate for at least 45 minutes before leakage through the seal will occur. Once air pressure is restored, the seal will return to normal operation.
The sealant reservoir contains approximately 2 inches3 (33 centimeters3 or 60 grams) of sealant, which is sufficient for about 18 months of operation in most cases. If the reservoir needs to be replenished, this can be done while the pump is running and takes about three minutes.
The proximity rod can be fitted to engage a switch that activates a signal to an attendant when/if sealant has to be replaced in the reservoir. The signal can be a message to a cell phone, SCADA or other monitoring device. This allows a time period cushion of about three weeks in which the sealant reservoir can be replenished before the supply is exhausted.
Since the pressure on the flexible sealant must be maintained slightly above pump stuffing box pressure, calculating the stuffing box pressure, which is different for each pump type, is important. In many cases, the stuffing box pressure is the same as the pump suction or discharge. Some exceptions are: