Rix SA Nitrox Compressor modifications - long!

["Nevil & Rachel Adkins" <adkins@xxxxxxxxxxxxxx>]

Further to my earlier post see the below communication from HSM Engineering.
It is long but has loads of info regarding these compressors.  My memory
failed me slightly - the max normal O2 for continuous blending is 40% not
50% as I stated earlier.
Cheers
Nevil

-----Original Message-----
From: iain <hsm-engineering@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
To: atkins@xxxxxxxxxxxxxx <atkins@xxxxxxxxxxxxxx>
Cc: anwyl@xxxxxxxxxx <anwyl@xxxxxxxxxx>
Date: Wednesday, 26 January, 2000 4:46 PM
Subject: Rix SA Nitrox Compressor modifications


>
>Thank you for your enquiry re Rix SA-6 units. The normal build
>specification rates these compressors suitable for air only.
>That is they are an oil free oil less breathing air compressor.
>
>However in Europe we supply a modified SA-6 unit suitable for nitrox 32
>or 36 percent up to 300 bar. Also a 40 percent 250 bar rated compressor.
>The same unit on air is rated 350 Bar for diving cylinder filling.
>
>Some of the major modifications are:
>1st stage piston, material change to hard anodize
>1st stage gasket, material change (02 compatible)
>1st stage piston liner, new size dims, ground to slip fit
>1st stage outlet, material change to stainless steel
>2nd stage head, material change (Stainless steel)
>2nd stage seal, change from Viton to Teflon
>2nd stage steel valve, material change
>2nd stage piston liner, new size dims, ground to slip fit
>2nd stage inlet, material change to stainless
>2nd stage outlet material change to stainless
>3rd stage ditto as above
>Bearing  swash plate x1, lubicant change from teflon to polyurea
>Bearings main x2, ditto
>Bearings piston x3, ditto
>Bearings Thrust plate x2, change to lithium HT
>Bearings main, dims change
>Lubricant seals, change to tribolube 16.
>Lubricant threads, ditto
>Relief valve 3rd stage, change to Nupro stainless ported relief
>
>All of these compressors can be used for air up to 350 bar cylinder
>filling and to 250 bar for bank filling.
>The increase in oxygen percentage does require us to reduce the final
>recomended pressure due to heat,
>Please note that some of the parts used in the standard air compressor
>range will react with pure oxygen.
>
>The higher the pressure the hotter the gas temperture becomes and the
>more work the compressor has to do. However with an inert gas such as
>air it matters little, the higher the discharge temperature will only
>reduce the life of the seal rings. However by increasing the oxygen
>percentage together with the pressure increases the risk of ignition.
>
>The compressor is air cooled by a fan driven at 1500 RPM the cooling
>effect from this process can only be improved if the ambient air
>temperature is reduced (not the case in Oman) or if the outlet gas
>pressure is reduced, or both.
>Not withstanding other cooling methods we have used in the past
>including coiling the outlet coils from the 1st and 2nd stages within
>water jacket coils connected to glycol chillers. This method is more
>cost effective with larger machines 30 cfm and above.
>
>In order to answer your question what is is the highest nitrox mix.
>In the factory UK we began with 32 and 36 percent units for both partial
>pressure and continuous blending and would test at 40 percent. As the
>demand for nitrox increased some of our customers started using the
>units to 40 percent. We responded by the addition of some material
>changes (some of which are listed above) and in increasing the testing
>gas up to 50 percent.
>In the UK some units have been used up to 80 percent and would require
>us to test on pure oxygen. This we would not do because of additional
>engineering requirements to the design of the compressor. For example
>the compressor is designed with a certain fixed compression ratio that
>is achieved by the diameter of the pistons in relation to each other and
>the length of compression stroke. The RPM of the compressor also has
>effect on the charging rate, compression rate and so to the heat
>generated. The higher the gas temperature due to compression ratios
>piston speeds, stoke speeds the higher the risks of ignition due to
>adiabatic compression, spontaneous ignition of particulate matter.
>
>As most divers air compressors are sold on price with little regard for
>conservative ratios most are high ratio with small high speed piston
>strokes. As opposed to the conservative low speed industrial oxygen
>compressors.
>
>Another point would be that of operator safety. All oxygen compressors
>in the UK are protected by a blast shield or a wall around the
>compressor but NOT enclosing it completely. The charging is done remote
>with the bottles in or surrounded by a presure blast wall or shield.
>Apart from the compressor ratios and temperature considerations we also
>have to ensure captive safe venting of all relief valves and piston
>leakage due to wear. Oxygen cannot be vented into a room.
>
>On the subject of compression and cooling the use of heliox.
>Pure Helium 98 percent diving quality dry to B.S. 4275 -60 relitive
>humidity will produce as much heat at 60 Bar than air does at 300 Bar.
>We have been working a solution to use the compressor as a scavaging
>pump for helium cylinders ie down to 2 bar bottle pressure with an
>external cooling chiller system but have not certified the unit yet.
>
>On the subjet of electrical power in order to compress 6 cfm to 350 bar
>the unit will need 5HP and at 415 volt 3 phase would draw 7 amps.
>However the same unit at 240 volt 50 HZ will draw around 25 Amps.
>This would require you to install as you would a cooker with 4mm copper
>wire protecting with a 40 amp circuit breaker.
>An alternative for Oman would be to reduce the rpm of the compressor run
>cooler and use a 4 HP 18KW electrical motor. This may be a useful
>solution. The 240 volt 50 Hz we would prefere to offer a 4HP unit.
>
>On the subject of maintanance Each piston is made from solid lubricant
>PTFE. Each acts as both the piston seal and a means of lubrication.
>Tribology that is wear friction and lubrication are all involved in the
>dynamics of the compression stroke. The reduction and wear of the
>plastic sealing ring is sacrificed on order to ensure clean oil free oil
>less gas purity. In our oxygen compressors oil less is essential and so
>oil less technology is used. For pure air requirements the ability to
>ensure breathble air without oil is increasing.
>
>The ring material for nitrox compressors is different to air compressor
>units. For nitrox and air in the UK we use a polycomp material. The USA
>builds the SA-6 only for air and uses carbon rings.
>
>Each stage 1st 2nd 3rd rings wear at different rates and requres
>replacement. As each ring fails to retain pressure a popping sound and a
>reduction in pressure or compression rates alerts the operator.
>Each piston is removed by one bolt and takes around 20 mins to replace.
>The average for the 1st stage is 800 to 1000 hours. The 2nd stage 500 to
>800 hours. The 3rd stage running over 250 bar replacement at around 150
>hours on average.
>
>The figures I supply are very conservative. I have just returned from
>the DEMA exibition (Diving Equipment Manufactures Association) a
>customer in Chile uses an SA-6 at 3600 psi and get 600 hours on the 3rd
>stage. In addition to lubrication as per the manual he strips down and
>internally cleans the unit every 30 hours.
>
>All the repair and maintanance can be done by yourself. The unit was
>designed for the military to be serviced in the field with the minimum
>of tools only a torque wrench with sockets, set AF and metric open/box
>spanners and a set of imperial/metric allen keys are required.
>Lubrication of the bearings is not required as they are all self
>lubricated ( replace after 800/1000 hours)
>The rose head piston bearings require cleaning and new polurea lubricant
>in the chamber every 25 to 30 hours. (replace after 800/1000 hours)
>When each piston ring is changed the o-ring is also replaced. Each
>piston has also a back up rider ring to ensure the metal piston never
>touches the metal piston liner. These should be replaced every second
>piston change or after 800/1000 hours.
>All bolts should be lock tight after correct torque.
>I will fax price list on spares.
>
>The compressor heads also require service.
>The 1st stage head on removal will require replacement of the gasket and
>a o-ring seal at mimimum. The inlet and outlet valves are in stainless
>steel and will require cleaning with a metal polish after 800 hours.
>Ensure the valves are oxgen cleaned before re using we have a written
>procedure for this process if you are using on nitrox.
>All parts we supply are cleaned for oxygen service however you are
>required to ensure no particulate matter remains in the tubes and coils.
>Clearly some items for cleaning are not suitable for high oxygen
>concentrations and must be removed. Some cleaning items ie wire wool are
>most dangerous in oxygen rich enviroments. We have tested all of our
>soft material, cleaning and lubricating items with oxygen in a process
>known as bomb testing and can report on the auto ignition points for
>most materials used.
>
>The 2nd and 3rd stage valves are cheep enough to replace together with 3
>o-rings in each stage every 500/800 hours.
>
>The use of this modified compressor for blending is of great interest.
>You do not explain how you intend to supply the oxygen, HP gas in
>bottles. or low pressure from a liquid LOX tank or indeed to manufacture
>the oxygen from either a membrane or pressure swing absorber PSA plant.
>
>Divers mostly use high pressure oxygen cylinders as the source of
>supply. your divers cylinders provided they are oxygen clean and have
>not been in contact with oil lubricated air can be filled by partial
>pressure or decanting.
>The HP decanting oxygen cylinder bank will fill a number of smaller
>divers cylinders to 40 bar. Once the oxygen bank is at 40 bar then
>filling by continuos blending is required.
>
>The oxygen is reduced from an inlet pressure of 40 bar with a small
>Tescom 44/2200 series regulator set with an outlet pressure of 4 bar.
>The regulator is also suitable for a high pressure inlet of 200 bar.
>
>The oxygen is connected into a Dwyer or similar flow meter. The flow
>meter can be graduated in any gas (air oxygen nitogen) or fluid (gas or
>water) or even simply graduated in a mm scale it does not matter
>provided that the accuacy is assured by the quality of the flowmeter and
>the length also that the maximum required percentage coincide to the
>upper graduations of the flowmeter.
>HSM supply a number of flowmeters for this dependent of flow and
>percentage. Certified and graduated in oxygen calibrated litres per min
>cost trade about 130 pounds sterling
>
>Remember that it is percentage of gas that you are most concered with
>not the flow. For example for a petrol compressor each time the unit is
>used the throttle could be placed at a different position giving a
>different output. On electric units any belt slip reduces the flow.
>
>The flow meter output is connected to the inlet hose of the compressor.
>In order to improve accuacy the intake gas velocity is reduced simply by
>the use of a larger diameter air intake pipe 18 inches long 100mm in
>diameter. The oxygen flow is introduced in the larger pipe and is drawn
>into the normal air intake hose of the compressor (3/4 inch diameter)
>
>In order to assure gas percentage a small bleed is taken from the high
>pressure outlet at 1 to 2 LPM into an oxygen analser. Once the
>percentage is assured the oxygen flow required can be marked on the
>flowmeter at say the 32,36 and 40 percent marks.
>
>During each operation of filling either the bank or a divers cylinder
>the compressor is started up on air only, with the bank or diving
>cylinder turned off and the oxygen supply turned off.
>Once pressure is obtained and the small flow bleed to the analyser is
>showing 21 percent then the oxygen is turned on and the flowmeter set to
>its mark or is calibrated with the analyser to the desired percentage.
>Once the percentage required is showing steady on the analser then
>either the bank or divers cylinder is opened for filling.
>
>On some of our advanced blending units we use a high and low leval
>oxygen alarm system with the high alarm point being wired to the cut out
>relays in the compressor to shut down also this is connected to a
>secondary oxygen solenoid valve that automatically shuts off the low
>pressure oxygen flow in the event of oxygen increasing percentage as a
>safety feature for remote automatic operations.
>
>On manual systems you must ensure that procedure is followed and that
>after the cylinder is full and switched off then the oxygen is switched
>off and the compressor is only switched off when the anayser reads back
>to 21 percent. On a manual system if you switch off the compressor
>before the oxygen then pure oxygen can be in the compressor on restart.
>
>I have read some nonsence from the sports training agencys regarding
>blending especially when they advocate the use of oil lubricated
>compressors, Stipulate an allowable oil concentration with nitrox,
>encorage a double or any number of filter process, and with the use of
>food grade oils seem to encorage a choice of process with the compressor
>operators. Partial or continuos, oilfree or oil lubricated, and have
>reduced the real explosive effect of oxygen with students to encorage
>training sales with the minimum of invested plant and equipment.
>Before you purchase a nitrox system I would reqire more information
>regarding the site location of the compressor (building commercial
>retail factory school), What it was next to (flamable material,
>classrooms, living accomodation,) The duty cycle required for the unit
>and the qualifications of the operators TDI ANDI or IANTD to get some
>idea of your risk training perception.
>
>Any idea you have for location should also involve cooling and
>ventilation considerations
>
>On the subject of pressure gauges any gauge used should be suitable for
>oxygen with the dial printed Oxygen use no oil. A blending gauge
>typically is of a large diameter 5 to 6 inches in diameter to ensure
>scale reading and is calibrated to 0.25 percent. The needle pointer is
>knife edge type not pointed and is painted red on the back edge. The
>dial is usually marked dual scale, Bar and PSI with 2 Bar divisions. The
>dial has a mirror reflex dial to see the back of the red pointer and
>ensure parallel reading. The accuracy of 0.25 percent. HSM provide a
>panel mounted 3 hole fixing unit for aroud 250 pounds trade
>
>Kind regards
>
>
>
>Iain Middlebrook
>--
>iain

-=-=-=-
[   Scuba-UK: Details @ http://www.drogon.net/scuba-uk-ndg/              ]
[ To leave the list, send "unsubscribe scuba-uk" to majordomo@xxxxxxxxxx ]