Lube oil system reliability-Kindly guide 2

[Raju1980]

Dear All;

Kindly guide me to decide what is the correct stand in the following issue?

For our new Chlor Alkali plant we are installed chlorine compressor of M/s Siemens make

940KW,10088 Rpm, 10048 nm3/hr flow 3 stage centrifugal compressor driven by electrical motor.

Oil system main oil pump shaft driven 188l/min and pressure of 5 kg/cm2

Auxilary oil pump same capacity as main oil pump but motor driven.

My concerns:-

1.In case of stopping of compressor if auxilary lube oil system malfunction wheteher lead to bearing failure.

The answer from the vendor is coast down time will be taken care by the shaft driven pump if auxilary lube oil system malfunctions.

Whether the main oil pump will supply oil till stand still? Whether this is checked any stage or how can we rely on that?

2. Incase total lube oil sytem failure as per API 614? how the failure is taken care.

Vendor says the double failure is not taken care in their design and that ocuurance is unlikely and our preference for run down tank/ emergency pump they are not accepting

What shoul be our stand?

3. how reliability of auxilary system can be ensured while compressor running since logic is that if main ol header pressur falls below 2.3 kg/cm2 auxilary lube oil pump starts and if falls below 1.7 kg/cm2 compressor trips.

4.Quote from API

When specified, an emergency oil pump shall be furnished to allow safe shutdown without damage to the equipment in the event that both the main and standby pumps fail.

The purchaser will define the requirements for safe shutdown.
Can we follow this stand?

5.“ Definition of emergency oil pump: A separate oil pump that provides adequate pressure and capacity to permit safe shutdown of the equipment when the main and standby pumps are
inoperable.”

API doesn’t says electrical failure/main oil pump shaft driven; regardless of condition this is arrangement for safe shutdown of the equipment when the main and standby pumps are
inoperable.

What is the justification for the deviation emergency oil pump is not applicable or not needed?

6.Lube-Oil Rundown Tanks

1.9.2.1 When specified, a separately mounted emergency lube-oil rundown tank (either atmospheric or pressurized) shall provide oil for the coast-down period specified by the
Purchaser.

Kindly guide

 

[BigInch]

Will they do a shutdown test and guarantee performance for life?

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[JJPellin]

As a general rule, a shaft driven oil pump on a critical machine is a bad idea. I would normally not allow this arrangement for a number of reasons.

The auto-start protection cannot be properly tested. There is no way to “trip off” or shut down the main oil pump to test whether the standby pump will auto-start and come up to speed fast enough to avoid a trip of the main compressor. This is a test that we would normally require. It cannot be performed if the main pump is shaft driven.

The reliability of a shaft driven pump tends to be worse than a pump that is independently driven by a motor or steam turbine. We have seen many failures of the couplings on shaft driven oil pumps. They are sometimes metal-on-metal designs that are lubricated by the oil running out of the adjacent gearbox bearing. In some instances, we have had to add separate lube lines just to lubricate that little coupling.

In one installation where we have a shaft driven main pump, we added a second independent auxiliary pump in case the first auxiliary pump fails. But, this requires three pumps with two drivers and a more complex control system. It adds complexity and maintenance cost. I don’t like this arrangement.

If I was forced to accept a shaft driven main oil pump, I would try to reduce my risk by adding a run-down tank or at least a well sized accumulator to provide additional oil during coast down. And, as BigInch suggests, I would ask to have the system tested for coast-down with no auxiliary oil pump. They could complete this test during the performance test of the compressor. The bearing inspection following this test should allow you to verify that you can coast down without damaging the bearings.

I assume that the shaft driven pump will run off of the gearbox. I would not allow the shaft driven oil pump to be coupled to the motor shaft under any circumstances. In the two installations where we have this configuration, the problems have been greater. In one instance, the oil piping was burst from overpressure when the motor was bumped for rotation and ran in reverse. The configuration of the relief valve and the foot valve on the pump suction line allowed for a dangerous overpressure event when the motor was run in reverse with the shaft driven oil pump coupled to it.

So, my recommendations would be as follows. Eliminate the shaft driven oil pump. Build the system with two motor driven oil pumps wired to two different feeders to reduce the chances of a common failure. If that is not possible, add oil accumulators to provide additional oil flow during coast down in the event of a trip with no auxiliary oil pump. Require the manufacturer to test the coast down case and inspect the compressor bearings for damage.


Johnny Pellin

 

[CaracasEC]

Hi Raju1980,
We are almost on the precommissioning phase of an ethylene cracker plant that has these process gas compressors (charge gas compressor driven with steam turbine, tertiary refrigerant compressor driven with steam turbine and hydrogen/methane compressor driven by motor with a gear box). All of these were specified thru api 617 for the compressor, 612 for the steam turbine and 614 for the lube oil system. The lube oil systems were driven by a main oil pump (screw type) driven by a steam turbine and auxiliary oil pump driven by motor. For flexibility of operation both motor and turbine are capable of automatic start-up. Due to this one we have increase the the number of accumulators because of the main oil pump steam turbine starting will be higher than the motor (which is 4 seconds)once the steam turbine driven lube oil pump is on the stand-by. In addition to these, we have also an emergency oil pump (screw type) motor driven and a run-down tank. Once there is an electrical failure, the steam turbine driven lube oil pump will still supply lube oil to the bearing and also the run-down tank. During the mechanical running test of these compressor, bearing performance and bearing inspection test is part of api 617.

 

[Raju1980]

Thanks to one and all;

In case of us factory test already completed and the coast down test didn;t done or data is not available.

Wheteher mechanical test includes coast down test also as per API 617/614?

one more thing in commissioning stage, all depend on the contract between purchaser and vendor;Most of vendors have deviations from API and they claims that their system is better ! What is your view of deviations to API 614 that is our real problem as i manifested?

 

[CaracasEC]

Hi Raju1980,
1. API 614 1.4.7 Shaft-driven pumps may be provided only if approved by the purchaser.(As the reliability of shaft driven pumps is less than the reliability of motor pumps then you have the right to disapproved).
2. API 614 1.4.10.2 Each pump shall be capable of transient oil requirements. 1.8.2 The system delivery pressure shall be maintained above the shutdown switch settings during standby pump acceleration or for at least 4 seconds for motor driven pumps.(Accumulator will give high reliability of the system because you have a low pressure trip on your lube oil system below 1.7kg/cm2)
3. During electric power failure, the rundown tank design should be capable of lubricating your bearings.

Regards,

 

[Raju1980]

Dear CaracasEC;

I gone throgh the API 614 1.4.7 you are correct; how we can explain reliability of shaft driven pump is less than that of electric driven any supporting documents or standards; since the manufacturer saying that the " The design is safe with to indipendently driven independently operatied equipment (electrically and mechanically driven pumps, so failure of main and back up equipment is only hypothesis"






'

 

[rakuday]

Raju1980,

Shaft driven main oil pumps are reliable and they are commonly used in Integrally geared centrifugal compressors by many manufacturers. As long as the shaft rotates, the oil will be pumped to the bearings. As the speed reduces the pumping capacity also reduces, so is the requirement of oil for the bearings. Regarding the reliablity of the AOP and MOP, we did a few tests for centrifugal air compressors, in which the complete power to the machine was cut-off, including the AOP. The machine did not have any issues, the bearings were investigated and found no issues.

Manufacturers usually do not comply partly with any standards / codes. If the compressor is a standard offering, then they avoid to include any modifications, as it beocmes a non-standard compressor and from their end, the quantum of work increases. On the contrary if it is a fully coded machine, then they will meet the requirements of the code.

Accumulators are used if the requirement is low, for bigger machines, the oil requirement will be high and the accumulator size will be huge.

With regard to the codes, all manufacturers will have a speficied list of deviations from the code which are mutually accepted. The level of deviations will vary (level-1, level-2...) according to the customer requirement/acceptance.

One objective of the mechanical test is for checking the lube system performance, but i am not aware if it includes anything related to its failure.

 

[CaracasEC]

Hi Raju1980,
Based on the Material Requisition/Purchase Order, was it mentioned on your specification that the compressor shall be design, manufactured, inspected and tested as per api 617 (for the compressor) and 614 for the lube oil system? If so, then the manufacturer shall comply and any deviation have been mutually settled during the technical clarifications. In case of low lube oil pressure alarm, the AOP will cut-in, how will you inspect/repair the main oil pump? On the petrochemical i am working with, all of the critical equipment/unspared equipment are using main oil pump and auxiliary oil pump driven with motor. We have also boil-off gas compressor and nitrogen compressors that uses shaft-driven main oil pump, however they are spared equipment one running + one standby. Under api 614 chapter 2 special purpose oil system para 1.1.2 The design shall allow for transfer between and shutdown of the main and spare components of the system for maintenance without interrupting the operation of the system or the equipment the system serves. Shaft driven main oil pump are reliable to be used on spared equipment but as per api 614 for critical service/special purpose equipment flexibility of lube oil pumps is necessary.

 

[Raju1980]

Thanks CaracasEC & rakuday
rakuday u tested ur machine at site... To verify the coast down implication.......

So u r suggesting it is ok...the design.

caracasEC. I am agreeing to u r stand....but when it is advanced to a stage
The equipment is installed and ready to start in months...

What is u r advice .

As u know it is difficult to accept changes in this stage........


Can I ask,what will be u r stand...

 

[CaracasEC]

Hi Raju1980,
Ask for the experience list of this package with the same application, rating etc and the MTBR of the main oil pump.
Regards...

 

[Raju1980]

Thanks will do as per your suggestion and will update....

 

[rakuday]

Raju1980,

The machine was tested in a manufacturers R&D test bed. This was part of several other tests conducted on the machine when the power is switched off abruptly and to check the status of critical parts of the compressor.

With regard to the design of shaft driven MOP's, they are safe and reliable (in my experience, ofcourse )

 

[RotaryG]

Raju,

It appears that you are not in the design phase but almost in commissioning phase! Congratulations. The definition of the preferred configuration of the oil system starts with data sheets and material requisitions (or whatever you call in your organisation). Since you have gone past that phase of the project, there is precious little you can do in asking for a change of the configuration from the supplier.

Even though JJPellin explained the risks of shaft driven oil pumps, it is not all gloomy. There are many (and much larger) equipment such as gas turbines which have shaft driven main oil pumps and electric motor driven auxiliary pumps. The arrangement if engineered well should not be of concern particularly when the supplier can show you references of similar operating field installations. If you find they don’t have filed references, then you need a design audit of the shaft driven oil pump and its coupling. Also make sure that you hold operational spares for that pump as well. The design audit may also help you in determining the need for insurance spares for this pump.

Also, you should be able to check the operation of the aux pump while the compressor is coasting down and satisfy yourself that it kicks-in at the correct (as advised by the supplier) oil pressure set point.

In summary shaft driven mail oil pumps are used in larger equipment and your fears even though well founded can be rationally addressed without asking for a change from the supplier at this late stage.

 

[jbattershell]

I have designed many compressors with shaft mounted main oil pumps. I re-designed a number of them with better couplings since that was one of the failure points.

an electric motor driven aux pump will allow the compressor to start up with full oil pressure and will allow the compressor to continue to run if the main oil pump experiences some type of failure that is not upbrupt. but in most cases, if the main oil pump stops, the aux oil pump can not start to provide pressure quick enough to prevent a trip to the compressor. for those customers that want to prevent this, a small accumulator is the solution. the accumulator provides oil pressure for the time needed to get the electric pump up to speed and producing pressure (just a couple seconds or so).

some customers like to have a run down tank to provide emergency oil during an electrical power failure. this is really a belt and suspenders approach. all of the compressors I have worked on or designed, could easily coast down on just the main oil pump. I have tested this many times in the lab. the oil pressure of course drops with speed, but so does the requirement for oil pressure. by the time the compressor hits 10% speed, the oil pressure is nearly Zero but at this slow roll speed, so is the need for flooded lubrication.

as stated above, all of these conversations need to be had prior to building the compressor.

and, for the individual that bump started the motor and broke the pump and piping, I am sure there were instructions to de-couple the oil pump prior to bumping the motor. Or, it is also possible to order the oil piping with reverse rotation protection which will provide oil flow even if the pump is run backwards. check valves arranged will let the pump get suction flow and discharge flow in either direction of rotation.

 

[Raju1980]

Dear All;


Thanks for your valuable inputs.


Continuation to our discussion ;


API 614 forseen the even of total lube oil failure.

Both Main and auxilary pump failure and the run down tank and emergency oil pump come under that.

in this case if main oil pump fails to develop pressure and auxilary lube oil pumps develop pressure and after some time it trips.

There is no provision for lubrication.

also find the quotes from API 614.

3.11 emergency oil pump: A separate oil pump that provides adequate pressure and capacity to permit safe shutdown of the equipment when the main and standby pumps are inoperable.

1.4.3 When specified, an emergency oil pump shall be furnished to allow safe shutdown without damage to the equipment in the event that both the main and standby pumps fail.

The purchaser will define the requirements for safe shutdown.

1.9.2.1 When specified, a separately mounted emergency lube-oil rundown tank (either atmospheric or pressurized) shall provide oil for the coast-down period specified by the Purchaser.

API 614 1.4.7 Shaft-driven pumps may be provided only if approved by the purchaser.(As the reliability of shaft driven pumps is less than the reliability of motor pumps )

API 614 1.1.2 The design shall allow for transfer between and shutdown of the main and spare components of the system for maintenance without interrupting the operation of the system or the equipment the system serves. Shaft driven main oil pump are reliable to be used on spared equipment but as per API 614 for critical service/special purpose equipment flexibility of lube oil pumps is necessary.

how will you inspect/repair the main oil pump

So the option of Run down tank is desiarable.....