| Larry Bachus
(a.k.a. “The Pump Guy”)
I’ve read with interest your various articles over the years and have learned quite a bit from them. I work in sales at an independent pump rebuild shop. I’m wondering if you can comment on a situation we have with a client’s pump.
The application is municipal potable water. The pump is a horizontal split-case centrifugal pump, designed for 156-ft. head @ 4,500 GPM.
My client’s controls are set-up so the first step in the shutdown procedure calls for a downstream control valve to close while the pump is still running at full speed (1,750 RPM). There is no variable-speed motor, so the pump deadheads 30-45 seconds every time the system is shut down, with the resulting vibrations, rumbling, and cavitation noise.
The pump is currently packed. My client has requested we install split mechanical seals. We are unsure as to the suitability of split mechanical seals for this application and are looking for your thoughts and opinions.
Can you tell me if you feel that split-type seals (or any type of mechanical seal) would be appropriate for this application?
Thanks for writing. I’m glad you like the articles.
|A disassembled split mechanical seal|
It is definitely a good idea to convert a cold, potable water pump to mechanical seals, compared to packing rings. Generally, pump packing rings must leak to work properly. It is always good to keep the liquid inside the pump to conserve water. And split mechanical seals are a good option to conserve time and labor on installation and maintenance.
You say your client’s split-case pump delivers 156-ft. of head. If this is best efficiency head, the shut-off head (deadhead) is about 184-ft or about 80 PSI differential for water. Do you have the pump performance curve? Can you verify this?
I last installed some split mechanical seals into split-case water pumps in the early 1990s on the Caribbean islands of Barbados and Antigua. Those seals were good for 300 PSI maximum pressure. I’d say the maximum pressure rating on a modern split seal is higher today. So, the pressure generated by this deadheaded municipal water pump is within the pressure limit of a standard split mechanical seal.
It really isn’t good to operate a pump against a closed valve. The deadheaded pump stresses the pump internals, including the packing rings and the shaft bearings. It will certainly stress a standard mechanical seal, and it will stress a split mechanical seal. You can expect problems on a water pump that is consistently and frequently deadheaded without correction.
As I understand, your client deadheads their municipal water pumps as part of the shutdown procedure. Their process is designed so that the pump frequently operates up to 45 seconds against a closed valve on shutdown.
I’m no expert with process control, and I’m no expert with municipal water plant automation. Talk with the process control technician at the water plant. Maybe this delay can be reduced.
Think about your car’s cruise control. The cruise control cancels the instant your foot touches the brake pedal. The relay is through the electrical circuit to the brake lights. Why would you want a 30-45-second delay in this process at 70 mph?
Plant automation is outside my area of expertise. However, if I was the client, and my operating system was designed so that my water pump operates frequently against a closed valve, and I know I must deal with frequent premature seal failures and other pump problems, then I’d want two split mechanical seals on my split-case pump. Why? A split mechanical seal is easier and faster to change on a split-case pump than a conventional mechanical seal. How’s that for logic?
With conventional mechanical seals on a split-case pump, you must remove the pump’s end bearings and the motor coupling, and maybe even move the motor to change the conventional component or cartridge mechanical seals. You don’t have to do this with split mechanical seals.
Also, you must break the casing gasket seal and remove the top casing of the split case pump to change conventional component mechanical seals, or to change the sleeves if the pump is packed. Go get the “A” frame and a chain hoist! And, scraping off the old, vulcanized casing gasket is time-consuming and a real pain. (You may not have to do this with some modern cartridge seals.)
You don’t have to do these things to change/remove/install split seals. I say, “Let your customer have his split seals.”
One more thought …
The biggest issue with split seals is the tendency to remove the packing rings or the standard mechanical seal and install a split seal onto a sloppy, unbalanced, misaligned pump shaft with an eroded unbalanced impeller. I’ve seen mechanical seal literature promoting that you can haul-out the old packing rings and slap-on a split mechanical seal without rebuilding the pump’s rotary assembly. This is just wrong unless you like to sell and install split seals.
You represent the pump repair shop. Tell your customer you want to recondition the pump one more time before installing the first split mechanical seals. If he doesn’t understand, show him this article. Give the seals a chance to survive. Install the first split seals onto a straight, balanced shaft with balanced sleeves and balanced impeller(s) with new wear rings, new bearings, and align the pump shaft to the motor shaft.
Any mechanical seal (split or conventional) can develop a mysterious, premature leak for any number of reasons. Maybe the pump suffers a bout of cavitation-induced vibration. Maybe the pump runs dry and the seal overheats. Maybe a load of sediment or crystals passes through the pump and damages the seal faces. The split seal can be changed in about 20 minutes with a rebuild kit, leaving the pump assembled and in-line. A conventional seal will require partial or complete pump disassembly.
I know for a fact that most of those split seals we installed onto some water pumps back in the 1990s are still on and running in 2012. I spoke recently with the GM at the plant in Barbados. He called me for advice on locating a PD metering pump. He confirmed they are still using the same seals, with some rebuild kits, because we reconditioned his pumps as part of installing the first split seals some 18 years ago.
I hope this info is useful. Man, I love this business! Pumps aren’t glamorous like a new car or skyscraper, but pumps are really good for raising a family and paying the bills—even in a bad economy.
Larry Bachus (a.k.a. “The Pump Guy”)
Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a retired member of ASME and lectures in both English and Spanish. He can be reached at email@example.com.