Case Drains

Casing drains should be blanked or spaded.

check

Thanks for the replies, I couldn totally comprehend the replies. Am i right to say that you had wanted to include a check valve? I would also like to find out what are the consequences of not putting check valves int them. Thank you.

Pump Drains

The question:

"I am working on a project where, i am tying the case drain lines of two identical centrifugal pumps together and returning them to the reservoir. One of the C-pumps will always be in operation, while the other will be used as a back up and will only be activated if another pump goes down. I was wondering whether it is alright to just tie the 2 case drain lines together or should I put a check on each case port, to prevent the case oil from the operating c-pump to enter the case port of the centrifugal pump that is not in operation and directly go to the reservoir. Does it matter and what are the advantages and disadvantages of the two ways."

Other comments:

"The Prof"

"Casing drains should be blanked or spaded".

"lynlynch"

"check"

My answer:

First – I am almost 100% sure that this statement is not correct. "One of the C-pumps will always be in operation, while the other will be used as a back up and will only be activated if another pump goes down."

Every process plant that I have designed or been in had an Operating/Maintenance program for "Paired" Centrifugal Pumps. These programs dictated a each pair of Centrifugal Pumps would be on a 24 hour or 48 hour cycle for "Run" and "Rest". This meant that the operators in the Control Room would be required to turn-on the at Rest pump and bring it up to speed then turn off the pump that had been running. The purpose of this is to balance the wear of the pumps, keep the bearings lubricated and insure that the "Seals" do not dry out. Very often if there is a minor repair that maintenance needs to handle on one of these pumps they know they must get it done as soon as possible while the pump is in the "Rest" mode.

Second - The drain lines should not be tied together, they should be run directly to the reservoir individually. Whether you run them individually or tie them together is more a matter of the situation you have. Both ways the two lines from each individual pump must have a proper block valve for isolation during maintenance. This valve should be normally open during all normal operation including both the "Run" and "Rest" periods. As noted by "The Prof" it might also be prudent to install the ability to have "Spec" blinds in these lines but these would be only for positive isolation during maintenance otherwise they are in the open position.

Third – The reservoir will normally be at a lower elevation than the two pumps. Otherwise you will not have a gravity flow from the pumps to the Lube Oil Console. With this in mind you should not need a check valve in the return lines but you should be sure you route the two lines so that the place the lines join is not on the same level as the pumps. Make it at a lower elevation in the last run to the reservoir.

I do not understand the comment by "lynlynch."

"Second - The drain lines should not be tied together, they should be run directly to the reservoir individually. Whether you run them individually or tie them together is more a matter of the situation you have. Both ways the two lines from each individual pump must have a proper block valve for isolation during maintenance. This valve should be normally open during all normal operation including both the "Run" and "Rest" periods. As noted by "The Prof" it might also be prudent to install the ability to have "Spec" blinds in these lines but these would be only for positive isolation during maintenance otherwise they are in the open position."

GA, but I have a possible addition. I'd consider NOT having a common reservoir, but one apiece. Scenario: pump A goes down due to a bearing failure and adds metallic particles to the lubricant, or loses a seal and DUMPS the lube. Pump B, when it is pressed into service, has contaminated lubricant in the first instance, or none in the second. If each has a separate reservoir, this can't happen. It may also simplify the ability to isolate for repair / maintenance tasks.

Ron,

Excellent recommendation, however a piper is not often in a position to influence what the Pump Engineer specs for the plant.

My understanding of the question was that it referred to "casing drains" which to me are the drains from the bottom of the volute or diffuser casing. They are used to drain the product from the pump prior to decommissioning for maintenance. As such they should always be blanked or spaded when the pump is in operation.

If the OP was indeed talking about something to do with lubrication then my answer is not valid.

As for changing over pumps on a 24 hour "run - rest" cycle I could not disagree more. Change overs are a risky time for pumps and should be avoided. The "standby" pump should be checked regularly and turned in order to ensure that it will be available for use when required. (For the reasons that you state plus Brinelling, rotor bow etc). The concept of "balancing the wear" is also an outdated (though very common) philosophy in my opinion, because if you actually get this right you will have two pumps that will fail on the same day. Ideally what you want to do is run one pump for its full maintenance life cycle (say 60 months for a modern process pump) and have 60 months life available to you on the spare pump for when the running pump fails or needs attention. The art is to keep the standby pump in good condition.