Equipment Redundancy

[dilip1949]

For my Compressed Air Plant I have used 3 X 50% Air Compressors as this has been specified by the ultimate Owner. Capacity of each Compressors is selected as 200 cu.m per hr.

My question is if required can I use the Compressors at 3 X 100%. What are the implications. What is the significance of 3 X 50% or 2 X 100%. Is their any relation to the design output of the equipment?

 

[TD2K]

3, 50% compressors would imply your demand is about 400 cu. m/hr (2 compressors are running, 1 is spare) If you run all three, you produce approximately 600 cu m/hr without getting into details as to how the discharge pressure might change and that effect on compressor output since there is no information to even try and quantify that.

My first question would be can your distribution system handle the additional air flow to user or ursers? If the air is dried before it's used, can your air driers keep up with the additional flow?

 

[BigInch]

Of course there is a relation. Given a required flow capacity of the system = Q , then 3 x 50% means you will have 3 pumps, each with a capacity of Q/2. Thus only 2 pumps will be operating when at full capacity; one will be stopped in case of malfunction of one of the other two. It could be immediately started to resume full capacity operation at Q. It would also be possible to easily operate at a capacity of Q/2, perhaps as low as 50% of Q/2 and as much as 65% of Q/2 with reasonable efficiency, using only one pump. 2 x 50% (Q/2) pumps should be relatively easy to operate at reasonable efficiency from 65% to 110 - 120% Q.

It follows that 3 x 100% = 3 machines, each with a capacity of Q, would imply one pump running, with 2 machines on standby. That would only be specified for an operation that required something like 99.995 reliability, where even if two pumps were not working, you would still be able to start #3 and operate at 100% capacity. Most operations would typically not need that reliability and could easily withstand operating for a short time at 50% Q, until a pump is repaired, but if you need almost perfect reliability at 100% capacity, 3 x 100% is what you would need to order.

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

 

[Artisi]

Think the OP has a problem with terminology, it reads to me as though the OP thinks each of the compressors in running at 50% of its capacity, not 50% of the plant requirement - "can I use the Compressors at 3 X 100%"

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[dilip1949]

Dear TD2K - Under normal operating conditions I will use only 2 compressors. I have 2 X 100% dryers so each dryer can take the output of 2 compressors. Compressed Air will be required for both Instrument Air and Service Air. For Service Air I am using a Diversity Factor of 33.33%. In only rare cases for a short time I may need to bring the third Compressor in service. Suppose I design the distribution system to handle output from 3 compressors (assuming the third compressor is not used to full capacity); am I safe in my design. This I am doing to optimize my system to bring down the CAPEX.

Dear BigInch - your logic is well understood. As a generalized scenario if I have a short time demand, can I use the third unit at say 50% when the other two are being used at 100%. Is there a statutory regulation that the third unit should not be brought in service except under breakdown / maintenance condition of the one of the two operating units? I will design the distribution system to take the extar output.

Thanks TD2K / BigInch for your help. I find the forum really useful. Regards.

 

[dilip1949]

Dear Artisi - The Plant demand is varying. Currently it is estimated as 400 cu.m/hr. So each of the three compressor is designed to deliver 200 cu.m/hr. Now for a short time if the demand goes as high as 500 cu.m/h can I bring the third Compressor to deliver the extra 100 cu.m/h.The Dryers each are designed for 400 Cu.m/h (2 X 100%) - so the extra 100% I will get from the second dryer. I will be violating the Basic Design of the System but is it permissible or is it against statutory requirement or violation of Contract?

 

[BigInch]

That is 3 x 50%. No problem with terms.
Three compressors running and equally sharing load at 83% of their individual design load should be possible. Whether running 3 compressors to make that 500 m3/h will be allowed by contract, we don't know. It depends on how your contract treats that 500 m3/h load. Is there a stated capacity of 500 m3/h that must be meet by running 2, and only 2, compressors in the contract, or is the 400 m3/h defined as the one and only maximum, or design, flowrate? Are there any other provisions in the contract that in any way require more than the 400 m3/h flowrate?

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

 

[CaracasEC]

Hi, what type of compressor is in question?Dynamic or positive displacement?
Regards....

 

[dilip1949]

Dear BigInch - All that the Contract specifies:
"A complete compressed air plant shall be provided to meet all instrument and service
air requirements. The air compressors shall serve both instrument air and service air with priority given to instrument air and essential service air in cases of very low header air pressure. All compressors shall be of the same capacity. Normally, two (2) air compressors
shall be operated to meet the compressed air normal continuous demand for the
plant instrument air and service air system. The remaining air compressor unit shall
be on standby mode. The arrangement that determines the highest capacity of the
individual compressors shall prevail". Sizing of the Compressors is Contractor's responsibility. The values I quoted are just examples.
(@CaracasEC) The Compressors are electrical, direct driven, oil free, rotary type.
What I conclude from all the discussions that irrespective of the Plant type (a pump system, a compressor system or anything else); if 3 X 50% redundancy is specified - there is no harm technically to use the third unit and it will not be detrimental to the balance of the system if properly designed. Under normal circumstances the redundancy should not be breached. But extreme emrgencies if used should not be a matter of concern.

 

[BigInch]

Nothing technically wrong, but contracts sometimes have many different meanings. It sounds like no rigid criteria was actually specified. Propose your solution to your client and see if that's what they had in mind.

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