Dear Art,
I take the liberty to reproduce the contents of your reply in Excel sheet for benefit of all. Being a naive in the field of reciprocating compressors ( a lot of other things for that matter), I misjudged the 1500 RPM as that of the compressor , which in fact is the RPM of the driver. I misjudged the amount of data required too, again blame my ignorance.
The compressor as such has a RPM of 263, quite within the range you prefer. I have provided few more details in the attachments and also cleared some of the doubts in the reply below.
● The compression ratio of each stage is the product of the ABSOLUTE discharge pressure divided by the ABSOLUTE suction pressure - it does not relate to the gauge pressures.
● Note that when the 1st stage suction pressure of the 90% H2 flow is increased by 5 kg/cm2 (71 psi), the compression ratio increases dramatically.
● No flow diagram, P&ID, or process description is given. It is assumed that the feed stream is supplied evenly and in a controlled manner. The feed available is in abundance. in a sense the suction pressure is controlled by sending the excess to elsewhere, so no feed starvation is there.
● No information is given on the capacity control(s) and it is assumed that the compressor is allowed to run at rated speed and capacity in each stage. There is no capacity control in place and the compressor works in 100% load all the time.
● It is assumed that the 2nd stage discharge pressure of the compressor is held constant by a back-pressure control valve on the discharge stream. IT is rather controlled by system pressure of hydrotreater it is supplying to, which infact is controlled.
● No information is given on the cylinder lubrication feature - lubed or non-lubed; it is not known if this compressor is a new purchase or used (from another application?). It is a lube cooled compressor and in use for last 17 years which is due for overhauling as reported by maintenance guys.
● Although the service is not severe, it is taxing on a machine that is double-acting (on both stages?) and running at 1,500 rpm. This is considered as an excessive, high speed - especially for Hydrogen, a very small molecule. RPM is 263, double acting in both cylinders.
● A manufacturer's detailed data sheet (such as API 618 Data Sheet) is needed to evaluate the compressor's rated operation and present results. Could manage the detail Engg consultant's datasheet and few details from manufacturer's installation guide. hope this suffices.
● A discharge temperature on an oil-lubricated cylinder should be around 300 °F (148 °C) as recommended by API standards. This temperature is normally used as an alarm point and 350 °F (175 °C) is the shutdown temperature. These alarm points are related to the limits of the lubricating oil used - not to the machine itself. The potential hazards are oil ignition and explosion (in air service) or oil coking and sludge creation.
● The capacity or size of the compressor and driver are not given. Capacity is 188 Kg/hr
● I would normally specify a 3-stage, oil lubricated compressor for this application running a conservative 300-400 rpm. This would reduce discharge temperatures (and wear).
● Depending on the type and present condition of the piston rings on this double acting machine, there might be ring wear - particularly in the 2nd stage - that is causing some re-compression and elevating the discharge pressure(s). This is hard to speculate without past compressor operating records and history. Certainly, a super-high rpm of 1,500 rpm is going to favor fast piston ring wear as well as bearings and valves. That is the price you pay when you specify a cheaper, fast rpm machine. This type of machines are not meant for application as dependable process compressors. They are designed and employed out in the oil patch where they are depreciated at a very fast rate just as everything else in the oil patch. They are not meant for the usual 10-15 year operating life.
● More information is needed on the compressor: Make, model, type, cylinder sizes, rod size, configuration, capacity control, driver specifications, etc., etc. Its a Dresser Rand make
In addition to above, we intend to improve the jacket, bearing housing and seal cooling system of the compressor by means of providing Deminaralised water/ steam condensate with ethylene glycol in 50:50 ratio in a closed loop tempered water system. The makeup in the jacket cooling tempered water system is presently from cooling water which tends to choke frequently. We assume this would help improve the discharge temperatures.
Edited by Phukan, 22 December 2018 - 02:11 AM.