If you increase the clearance, the long-term reliability and efficiency of the pump will suffer.
We hope you have had a chance to enjoy your summer holidays. From New Orleans to Narvik, it’s hot out there, so be safe, and wear sunscreen.
In our messages, we frequently highlight how Vespel® CR-6100 does not seize and therefore allows you to reduce the clearance at the wear parts in your pumps: wear rings, inter-stage rings, throttle bushings, and center bushings.
Today we want to look at things from another perspective–negative effects which can happen to your pump when you increase the clearance at the wear parts.
What can happen when you increase clearance?
If a process plant has a problem with a pump seizing during operation or galling during commissioning, the traditional response has been to increase the clearance at the wear parts.
Although increasing the clearance might make the pump operable in the short term, there are several negative consequences from increased clearance.
|Hydraulic Effects||Mechanical Effects|
|Lower head||Reduced rotor stability|
|Lower flow||Potentially higher vibration|
|Lower efficiency–increased power consumption||Potentially higher shaft deflection|
|Higher NPSHR–greater risk of cavitation||Increased risk of shaft breakage|
|Higher motor load||Potentially shorter seal life|
|Need to run steam turbines at higher speed||Potentially shorter bearing life|
|Higher likelihood of needing to run pumps in parallel||Higher risk of motor over-heating or tripping from excessive load|
So, while you don’t want your pumps to seize, increasing the clearance can create some major issues. At a minimum increased clearance drives up the operating cost of the pump and likely compromises the long term reliability of the machine.
The Poster Pump
A while back, one of our clients had an 11-stage horizontal pump which was originally supplied with metal wear components. The pump seized soon after start-up, and the recommendation from the OEM was to increase the clearance. The pump seized again. The second recommendation was to use a “non-galling” metal alloy to address the problem. The pump seized again. The clearance was increased one more time. When the pump was started again, the overall pump vibration levels were beyond alarm limits. The multiple increases in clearance had resulted in a loss of rotor stability to the point that the pump was no longer operable.
The end of the story will be in our next email…
Until next time, if you have a pump in your shop which has galled or seized, contact Boulden to discuss an upgrade to Vespel® CR-6100. We will be happy to work through the details of the upgrade with you and we have material in stock and available for immediate shipment.
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