Volume II, Issue 1 NAVAL SEA SYSTEMS COMMAND March 1997

Transcription

1 Volume II, Issue 1 NAVAL SEA SYSTEMS COMMAND March 1997 s High Pressure (HP) and Medium Pressure (MP). The air compressor is the heart of any compressed air system. Introduction: Divers air systems have been designed using many configurations. Regardless of the system configuration, every diver s air system must have either directly or indirectly a compressor to supply breathing gas to the diver. All U.S. Navy divers air systems use either medium pressure (MP), high pressure (HP) or a combination of those compressors. Specific compressor models which may be used to provide divers air are listed in NAVSEAINST C, DIV- ING EQUIPMENT AUTHO- RIZED FOR NAVY USE (ANU List). Commands desiring to use a compressor which is not on the ANU list may request an exemption from the Supervisor of Diving (NAVSEA Code 00C3B) or may provide technical justification to the System Certification Authority (SCA) on why it should be certified as part of the Diver s Life Support System. This request must be supported with a compressor discharge air sample and a compressor capacity test. Types: Currently, all HP and MP compressors used to supply divers air are reciprocating piston type compressors. These compressors can be further subdivided into oil-lubricated and oil-free models. The conventional reciprocating, oil-lubricated compressor is the most commonly used type. This type of compressor uses a forced-fed lubrication system, similar to that found in an automobile engine. The principle elements include: oil reservoir; oil pump; oil temperature indicator; oil failure pressure switch; oil filter; and drilled oil passages in the crankshaft and connecting rods. From a contamination standpoint, oil-free compressors provide virtually contaminant free air. However, oil-free compressors initially cost more and require a great deal more maintenance than comparable oillubricated compressors. Through the use of backpressure regulators (required for HP compressors only), moisture separators and coalescing air filters, oil-lubricated compressors also provide breathing air well within acceptable contamination limits. Design: (High Pressure) There are several variations in the design, construction and method of compression of air in the manufacture of air compressors. The construction and design principles most common to diving systems are discussed below. Reciprocating HP compressors are multi-stage compressors having three to six stages of compression. A multi-stage compressor is shown in Figure 1. During the compression phase, each successive stage of the compressor increases its inlet pressure. The compressing element of a reciprocating compressor is made up of the cylinders, pistons and the air valves. The 1st stage of the compressor takes a suction from the atmosphere on the downward stroke of the piston. On the upward stroke of the 1st stage piston, the air is compressed and then discharged to the 2nd stage cylinder, which is now taking a suction by the downward stroke of its piston. This process is repeated in each successive stage. The air valves are used to prevent the air from flowing backward out of the cylinder to the previous stage on the compression stroke. Two types of valves commonly used in high pressure air compressors are Published by: NAVAL SEA SYSTEMS COMMAND From the Office of: Director of Ocean Engineering Supervisor of Salvage and Diving

2 2 Topside Tech Notes March 1997 Figure 1. HP Assembly. the strip-type and disk-type valve. The strip-type valve is used for the lower pressure stages. At rest, the thin strips lie flat against the seat, covering the air passage slots. When pressure on the seat side exceeds the pressure on the guard side, the thin strips flex, permitting air to pass through the slots. When the pressure equalizes, the strips unflex and return to their original position. The disk-type valve is used in the higher stages. At rest, the disk is held against the seat by a spring. As pressure on the seat side of the valve overcomes the spring pressure, the disk lifts, allowing air pressure to pass through the valve. The actual compression takes place as a result of a piston moving up in the compressor cylinder and decreasing its internal volume. Most modern HP compressors use a stacked cylinder configuration. This means that a smaller piston is physically attached to the top of a larger piston. The two piston assembly operates inside of two cylinders, one on the top of the other, as shown in Figure 1. Normally, the piston from the third stage is stacked on the piston from the first stage and the fourth stage is stacked on the second stage piston. This is done because the alternating stages compress the air concurrently. Stacking of cylinders allows air compressors to be more compact. Piston rings, similar to those used in an automobile engine, are used to form a seal between the piston and cylinder and prevent the compressed air from passing from the top of the cylinder to the crankcase below as the air is compressed. Oillubricated compressors use hardened steel, and chrome-plated piston rings. These piston rings also prevent lube oil from reaching the top of the piston and contaminating the compressed air. Oil-free compressors normally use piston rings fabricated from TEFLON or TEFLON COATED steel, which do not require lubrication. All HP air compressors must have a moisture separator, back pressure regulator and discharge filter installed at the compressor discharge. Design: (Medium Pressure) For diver s air breathing systems, medium pressure multi-stage, reciprocating type compressors are the type used. In operation, the intake air is compressed in two or more compression elements (stages) to reach a final discharge pressure as depicted in Figure 2. This compressed air is delivered directly to the diver s supply piping or to air stowage for later use. Reciprocating air compressors consist of a system of connecting rods, a crankshaft and a flywheel. These components transmit power developed by the drive unit to the pistons, lubrication, cooling, control and unloading systems. The compressing element of a reciprocating air compressor consists of the cylinder, the pistons and air valves. All MP air compressors must have a moisture separator and discharge filter installed at the compressor discharge. Operating Range: The difference between the pressures that cause a compressor to cycle on and off is called the operating range. The high pressure compressors in most diving systems are started manually when storage flask pressure drops to a predetermined pressure. When utilized in divers air systems, a high pressure controlling device stops the compressor when the pressure reaches maximum system working pressure. MP diving system compressors have a feature that will automatically start the compressor when the storage flask pressure reaches a predetermined low set-point. When the automatic feature is employed, the compressor restarts when the pressure falls to within ten percent of the system working pressure.

3 March 1997 Topside Tech Notes 3 Figure 2. MP Assembly. HP and MP compressors are capable of providing discharge pressures that are higher than the rated pressure of the system in which they are installed. To prevent overpressurization of the system, a high pressure controlling device (i.e., compressor cut-off switch, relief valve, etc.) is installed. Also, installed is a low pressure controlling device. In the Automatic Operation Mode, low pressure controlling devices control how low the system pressure may drop before the compressor recycles. Were it not for these devices, the compressor would require manual starting and stopping when the desired pressures are reached. If the high and low controlling devices are the same mechanism, the operating range is then referred to as the Differential Pressure Setting. Capacity Rating: Per NAVSHIPS Technical Manual (NSTM) Chapter 551, Compressed Air Plants and Systems, compressors are classified as low, medium or high pressure. High pressure compressors have a discharge pressure of 1,000 psig or higher. Medium pressure compressors have a discharge pressure between 151 and 1,000 psig. High pressure air compressors are capable of supplying divers air at pressures ranging from 3,000 to 5,000 psig. Therefore, HP compressors are used to charge HP air flasks and SCUBA cylinders, which are then used to provide air to the divers. This type of system is called a passive system because the compressors are not operating during diving operations. NOTE If the system configuration requires these compressors to be operating during diving operations, then they must be included within the scope of certification. Medium pressure air compressors supply divers air at pressures ranging from 200 to 250 psig. MP compressors are used to supply air directly to the diver s air system via a receiver. This type of system is referred to as an active system because the compressor must be operating during diving operations. Installation: High pressure and medium pressure air compressors in support of diving systems are permanently installed on ships such as ARSs, ASs and on some Intermediate Maintenance Activities (IMAs) and Naval Shipyard dive boats. Permanently mounted compressors are normally electric motor driven and portable compressors are normally diesel driven. High pressure air compressors may also be a part of a Surface Supported Dive System (SSDS) to provide secondary air. Medium pressure air compressors can also be found in SSDS. All compressors vibrate considerably when operative. Therefore, all compressors must be shock mounted and flexible hoses or flexible piping of some type must be used to connect the inlet and discharge of the compressor to the associated system piping. Location: Placement of a high or medium pressure air compressor within a ship or on a diving boat depends on several factors: type of drive unit required (diesel or electric); weather protection; physical size; cooling media; support equipment to include receivers, batteries, fuel, electrical controllers and length of intake piping; atmospheric contaminants (engine exhaust) and ease of maintenance. The surrounding atmospheric environment is extremely critical to compressor intake air. Intake air should be piped in from the weather. It is important to the oper-

4 4 Topside Tech Notes March 1997 ation and life of the compressors that intake air be as relatively cool, dry and clean as conditions permit. Filters are provided on all compressor intakes as the primary means of providing relatively clean intake air. In accordance with NAVSEA SS521-AA-MAN-010, Diving and Manned Hyperbaric Systems Safety Certification Manual, a dry type non-shedding inlet filter is required. Oil bath filters are not allowed in breathing systems. Other requirements (covered in Appendix A) include the piping of automatic and manual condensate drains. Consideration also must be given to safety items such as installing guards on rotating equipment and minimizing personnel exposure to noise. Maintenance: All planned maintenance, i.e., PMS conducted on systems within the certified boundaries shall be accomplished using only approved maintenance procedures, i.e., MRCs. Some MRC procedures for equipments within the certification boundary shall contain the following notes: 1. This maintenance involves certified equipment and applicable certification procedures must be followed. 2. For the purpose of sustaining system certification, use only renewable parts listed in the tools, parts, materials and test equipment block. These notes normally apply to maintenance procedures that would require the individual to enter the system gas train integrity or other life critical sections of the system. The individual performing the planned maintenance or work center supervisor issuing the procedures should maintain strict adherence to the approved command Re-Entry Control (REC) procedures. Corrective Maintenance: Corrective maintenance is the process of correcting faults or unsatisfactory conditions discovered during diving system operation or while performing PMS. Corrective maintenance is discussed in the manufacturer s technical manuals in the form of text, sketches, photographs and troubleshooting charts. Before any equipment is disassembled, determination must be made whether or not REC procedures are required. (See Appendix A for general REC requirements for compressor maintenance/repairs.) Re-entry occurs anytime that the boundary of a certified system is disturbed. This includes breaking a mechanical flange or joint, removal of a valve bonnet, repairing a gland or replacing electrical system components, i.e., pressure switches or electrical sensors. When equipment is disassembled for corrective maintenance, look for indications of abnormal wear, failure, carbon buildup, etc. This information could lead to updating or changing the PMS schedule of maintenance requirements for a particular compressor. Repairs: In accordance with NAVSEA SS521-AA-MAN-010, U.S. Navy Diving and Manned Hyperbaric Systems Safety Certification Manual, commands must notify NAVSEA 00C, in addition to normal CASREP Procedures, of any situation that impacts the operational capabilities of a certified diving system. This includes material casualties, emergency repairs or proposed repairs to any equipment within the scope of certification that are estimated to exceed 48 hours. The repair of diver s high and medium pressure air compressors must be done in strict, compliance with the manufacturer s repair procedures. Improper repair and installation of items such as stop valves, check valves, intake and exhaust valves can lead to accidents. The manufacturer s technical manual is the proper source for repair information. Material control for replacement of repair parts in a certified system is also regulated by the manufacturer s technical manual. Cleaning: Air compressor cleaning is often confused with air system cleaning. Unfortunately, oil-lubricated compressors force a small amount of lubricating oil out of the cylinders mixed with the diver s air. Downstream filtration removes these oils and other contaminants such as metal particles caused by internal wearing of moving parts. Air compressors shall be thoroughly cleaned when they are disassembled for repairs or during regularly scheduled overhauls. Components shall be removed from the compressor and cleaned. This cleaning shall be sufficient to restore the internal surfaces to the as new condition. Clean internal components and passages on the air side of the compressor with components disassembled as necessary. If hydrocarbons and/or other contaminants are known to have been introduced into the component or piping, a degreasing procedure shall be initiated. Citric acid (MIL-C-11029B) and/or Non-Ionic Detergent (NID) or Navy Oxygen Cleaning Compound (NOC) may be used as follows: Clean the internal components and passages in the water side of the compressor by brushing accessible surfaces with a stiff bristle brush and an air, steam or water jet. This process removes loose deposits and scale. Inaccessible surfaces and surfaces covered with scale that cannot be removed by this procedure shall be referred to an IMA. The IMA

5 March 1997 Topside Tech Notes 5 will acid-clean the components per NSTM Chapter 531(V-1), Desalination Low-Pressure Destilling Plants. Some smaller components may be cleaned in an ultrasonic cleaner utilizing NID or NOC. Process Instruction NAVSEA-00C4- PI-002, Cleaning Diving System Air Components with NOC, may also be used to clean compressor components. Ensure that all components are thoroughly precleaned prior to placing in an ultrasonic sink. Testing: The most reliable diagnostic procedure for determining a compressor s efficiency is actual testing. Testing referred to in this section assists the user in maintaining an efficient, reliable and certified system. This section is by no means all-inclusive nor is it intended to supercede other requirements such as PMS or manufacturer s technical manuals. testing may start in an IMA repair shop during the required run-in period after overhaul. Testing may also be used to troubleshoot compressor irregularities. The location of the testing does not relieve the user of any responsibility. The user must ensure that required tests are properly conducted. For HP air compressors, a backpressure regulator may be used. No compressor shall discharge air into a cleaned system before satisfactory air samples are obtained. NOTE Run compressor discharging to the atmosphere for 2 hours prior to taking air samples. This will eliminate most contaminates introduced during repairs. An air sample prior to overhaul of compressor should also be considered. Performance Measurements and Efficiencies: These are testing procedures used to determine compressor capacity. These tests are required at the following times: when scheduled by PMS; when determining pre-overhaul condition of the compressor; when determining post-overhaul condition; and, any time it is suspected the compressor is not performing satisfactory. NAVSEA 00C3 has currently incorporated Capacity Testing into divers PMS for designated H.P. compressors. The approved Process Instruction NAVSEA-00C4-PI-004, Capacity Testing, included at the end of this Tech Note is available for all other DLSS compressors. Tightness Testing: A running tightness test at system maximum operating pressure is required with the compressor running at the following times: after a compressor was physically separated from the system and reinstalled; after mechanical disconnection/disassembly of air passages, piping, fittings or inlet/ exhaust valves. All joints and any static or dynamic seal (i.e, valve stems and relief valve poppets), where accessible, require testing for leakage using Snoop (a soap solution). In accordance with NSTM Chapter 551, no leakage is permitted. Leakage is defined as bubbles or foam created by air leaking from the fitting or joint. Objective Quality Evidence (OQE): OQE is the documentation main-

6 6 Topside Tech Notes March 1997 tained to produce a historical record. Types of OQE documentation include an REC package, PMS documentation, test sheets, supply requisitions, written approval or other forms to support compliance to approved procedures. Responsibility for maintaining OQE is borne by the command initiating the work performed. In accordance with NAVSEA SS521- AA-MAN-010, U.S. Navy Diving and Manned Hyperbaric Systems Safety Certification Manual, Appendix I. For further information on HP and MP compressors contact: NAVSEASYSCOM (Code 00C4), DSN or commercial (703)

7 March 1997 Topside Tech Notes 7 APPENDIX A COMPRESSOR CORRECTIVE MAINTENANCE DOCUMENTATION REQUIREMENTS MATRIX The purpose of this matrix is to provide guidance to the user in obtaining the required objective quality evidence (OQE) documentation after corrective maintenance and/or repairs have been performed. The requirements stated in this Appendix contain the minimum requirements for OQE documentation for sustaining system certification. If any of these requirements appear to be unclear, it is recommended that COMNAVSEA- SYSCOM (Code 00C4) be contacted. COMPRESSOR H.P. Oil Pumping; Excessive Vapors in Air Supply See Note (3) Yes Yes See Note (6) Yes Knocks or Rattles See Note (3) Yes See Note (5) See Note (6) Yes Decreased Air Delivery See Note (3) Yes See Note (5) See Note (6) Yes Safety Valve Lifts See Note (3) Yes See Note (5) See Note (6) Yes Water in Crankcase or Rust in Cylinders Rec Required for Check Valve Replacement Yes See Note (5) See Note (6) Yes Excessive Automatic Loading and Unloading Failure to Unload When Stopped Condensate Drain Fails to Drain Automatic See Note (3) No Rec for Adjustment Only No Rec for Adjustment Only Rec Required for Replacement Part Only Yes Yes Yes Excessive High Operating Temperatures See Note (3) See Note (4) See Note (5) See Note (6) Yes

8 8 Topside Tech Notes March 1997 Failure to Reach Speed, Loading Engine Excessively See Note (3) Rec Required for Replacement Part Only Yes Abnormal Piston or Cylinder Wear See Note (3) Rec Required for Replacement Part Only Yes Yes Yes Air and/or Condensate Leakage at Drain Valve, First, Second, Third or Fourth Stages Repair or Replace Valve Yes Yes Noisy Operation See Note (3) Yes See Note (5) See Note (6) Yes Engine Starting Starting Difficult or Failure to Start See Note (3) Yes Starter Motor Does Not Turn See Note (3) Yes Slow Starter Speed See Note (3) Yes Defective Battery Replace Yes Discharged Battery See Note (3) Yes Blue-White Exhaust After Failure to Start See Note (3) Yes Little or No Exhaust Smoke See Note (3) Yes Engine Operation Engine Misfire See Note (3) Yes Engine Misfire Under Light Load See Note (3) Yes

9 March 1997 Topside Tech Notes 9 Engine Misfire Under Heavy Load See Note (3) Yes Low Engine Power See Note (3) Yes Engine Overheating See Note (3) Yes Engine Speed too Low See Note (3) Yes Engine Speed too High See Note (3) Yes Loss of Governor Control See Note (3) Yes Hunting Condition See Note (3) Yes Mechanical Knocks See Note (3) Yes Engine Fuel System Excessive Fuel Consumption See Note (3) Yes Fuel Knocks See Note (3) Yes Incorrect Fuel Injection Pump Timing See Note (3) Yes Defective Injection Nozzle See Note (3) Yes Excessive Oil Consumption See Note (3) Yes Diluted Oil See Note (3) Yes High Oil Pressure See Note (3) Yes Low Oil Pressure See Note (3) Yes Crankcase Sludge See Note (3) Yes

10 10 Topside Tech Notes March 1997 Engine General Poor Compression See Note (3) Yes Worn Connecting Rod, Bushings, Bearings See Note (3) Worn Pistons, Cylinders and Rings See Note (3) Broken Pistons or Rings See Note (3) Broken Valves See Note (3) Valve Burning See Note (3) Sticking Valves See Note (3) Cooling System Overheating See Note (3) Yes Engine Electrical Engine Cranks but Fails to Start See Note (3) Yes Engine Starts but Stops after it has been Running See Note (3) Yes No Water Dripping from Condensate Drain See Note (3) Yes M.P. Lube Oil System Shutdown Due to Low Oil Pressure See Note (3) See Note (4) See Note (5) See Note (6) Yes Loss of Lube Oil Pressure See Note (3) See Note (4) See Note (5) See Note (6) Yes

11 March 1997 Topside Tech Notes 11 High Pressure Problems High Pressure on First Stage See Note (3) Yes Yes First Stage Relief Valve Lifting See Note (3) Yes Yes High Pressure on Second Stage See Note (3) Yes Yes Second Stage Relief Valve Lifting See Note (3) Yes Yes Low Pressure Problems Low Pressure on First Stage See Note (3) Yes See Note (5) Yes Yes High Temperature Overheats See Note (3) Yes See Note (5) See Note (6) Yes High Temperature Switch Shuts Down See Note (3) Reduced Capacity Rec Required for Replacement Part Only Yes Output of is Reduced See Note (3) Yes See Note (5) Yes Yes Longer than Normal Time Required to Fill Receiver See Note (3) Yes See Note (5) Yes Yes Unusual Noise Clacking Noises from One or More Cylinder Heads See Note (3) Yes Yes See Note (6) Yes

12 12 Topside Tech Notes March 1997 Flat, Slapping Sound When Starts and Stops See Note (3) Yes Yes See Note (6) Yes Metallix Knocks While Unloading See Note (3) Yes See Note (5) See Note (6) Yes Unusual Vibration Entire Vibrates See Note (3) Yes Moisture Separator Drain Problems Air Discharging from Moisture Separator See Note (3) Yes Yes Moisture Separator Drain Not Dumping Automatically See Note (3) Yes Inability to Start Fails to Start See Note (3) Yes Inability to Restart Fails to Start After Recent Shutdown See Note (3) Yes Runs 30 Seconds then Shuts Down See Note (3) Yes Inability to Stop Does Not Stop When High Pressure Set Point is Reached See Note (3) Yes

13 March 1997 Topside Tech Notes 13 Does Not Stop When Stop Push-button is Pressed See Note (3) Yes Leakage Problems Oil Leaking Past Oil Wipers See Note (3) Yes Yes Yes Yes Air Discharging from Moisture Separator Drain See Note (3) Yes Adjustments and Alignments V-Belts and Pulleys Adjust or Replace Yes Yes Repair/Overhaul Cylinder Heads Overhaul/Replace Yes Yes Yes Piston and Rod Assembly Overhaul/Replace Yes Yes Yes Yes Piston Rings Overhaul/Replace Yes Yes Yes Yes Oil Wipers Overhaul/Replace Yes Yes Yes Yes Connecting Rod Overhaul/Replace Yes Yes Yes Yes Guide Cylinder Liners Overhaul/Replace Yes Yes Yes Oil Pump Overhaul/Replace Yes Yes Main Bearings Overhaul/Replace Yes Yes V-Belts Overhaul/Replace Yes Gas Packing Assembly Overhaul/Replace Yes Yes Heat Exchangers Overhaul/Replace Yes Yes Moisture Separators Overhaul/Replace Yes Yes Yes

14 14 Topside Tech Notes March 1997 Solenoid Drain Valves Overhaul/Replace Yes Yes Unloader Valves Overhaul/Replace Yes Yes Thermometer Replace Yes Shaft Seals Replace Yes Yes Pressure Relief Valves Overhaul/Replace Yes Yes Gauge and Gauge Piping Repair/Replace Yes Yes Valves Overhaul/Replace Yes Yes Yes Yes Notes: (1) Conduct satisfactory air sample IAW USNDM, Vol. 1, App. I. (2) Conduct flow capacity test. (NAVSEA-00C4-PI-004). (3) Repair or replace IAW Manufacturer s Procedures/Recommendations. (4) REC will be required if any portion of the system s gas train is required to be entered. (5) Air sampling will be required, prior to connecting compressor to the system, when repairs are involved to ensure final cleaning criteria has been met or exceeded. (6) Flow efficiency testing will be required whenever compressor capacity is in question.