fig: 62.1 - Patt. 3 Manual Air Pump
Air Supply Arrangements
1. In standard diving, air is supplied to the diver from one of three sources, a manual air pump, a power driven compressor, or an independent battery of cylinders previously charged to high pressure.
MANUAL AIR PUMP
2. Although power driven air compressors are being used on an increasing scale, the manual air pump, which is reliable a means of supplying air to the diver, is still extensively used. The standard pump is the Patt. 3 Manual Air Pump the arrangement of which is shown in Fig. 62.1. It is a two cylinder double-acting pump theoretically capable of supplying sufficient air for two divers operating to a depth of 33 ft, or one diver operating to a depth of 99 ft; in practice, however, mainly due to leaks, it is impossible to obtain a fully efficient pump and this latter depth is limited to 80 ft. If two pumps are coupled together they are capable of supplying sufficient air for one diver operating to a depth of 165 ft, difficulty will, however, be experienced in maintaining the pump revolutions for depths greater than 120 ft, and this latter depth should be taken as the normal maximum working limit.
3. The pump has two cylinders each of 4 in. in diameter and having a stroke of 7¼ in. With each revolution of the pump hand wheel, each piston makes an upward and a downward pressure stroke delivering air to the diver. Each cylinder is capable of delivering about 1/10cu. ft of air with each revolution of the handwheel. The pistons are 90 deg. out of phase with each other.
4. The two cylinders and their associated fittings are identical. Each cylinder is bolted to a pump mounting plate and the cylinder cover is bolted to the cylinder, both being metal-to-metal joints. The cylinder cover contains an inlet and an outlet non-return valve serving the upper side of the piston, an oil cup with shut-off cock and a central piston rod gland. The oil cup provides a means of lubricating the cylinder and piston leathers and the cock seals the oil channel after oiling; olive or neatsfoot oil only are used for this purpose. The piston rod gland is packed with a leather washer compressed by the gland cover which is bolted to the cylinder cover.
PISTON AND CRANKSHAFT CONNECTIONS
5. The piston comprises two cup leathers placed base to base and each containing a gunmetal block; the whole assembly is fitted to the end of the piston rod and is tightly clamped by a nut on the piston rod recessed into the lower block. After fitting, the end of the piston rod is burred over to prevent the nut from slacking back. It is most important that this burring is efficiently done as the time the nut is likely to work loose is when the pump is in action and divers are depending upon its efficient operation; the piston clearance at the bottom of the cylinder is very small and the results of a loose nut may well be serious.
A spiral spring is fitted in a recess in the side of each block and these bear out against the skirts of the cup leathers keeping them tight against the cylinder wall; they are assisted in this by the cylinder pressure which has access to the spring recess through holes drilled diagonally through the blocks from the block faces.
The piston rod is connected to the crankshaft by a connecting rod or 'sling'; the upper end of the sling is solid with the bottom part of a crankshaft coupling and the bottom end is fitted with a pivot pin inserted through a square block which is solid with the piston rod. The upper part of the piston rod above the connecting block is supported in a guide bearing fixed to the pump frame. No provision is made for adjustment of the connecting rod, and hence, the piston travel. The crankshaft is supported in bearings at the top end of the pump frame and is operated at each end by a handle attached to a flywheel fitted on a square sectioned end of the shaft. The top couplings of the connecting rods and the shaft bearings are all provided with oil cups. The two cranks on the shaft are at 90 deg. to each other.
6. Air to and from the upper side of the piston passes through the inlet and outlet valves in the cylinder cover respectively, while the underside of the piston is served by an outlet valve contained in a Lower Valve Group situated in the front of the pump mounting.
All four valves are identical and are interchangeable; each comprises a solid gunmetal valve stem and disc to which is fitted a leather washer secured by a retaining ring nut screwed to the valve stem. Each valve is held closed on a gunmetal seating by a spiral valve spring. All the valves are accessible through screwed plugs without any stripping of the pump.
The valve plugs are made tight with leather washers.
Air to the upper side of the piston enters through a perforated valve cap and the inlet valve. With a pressure stroke on the upper side of the piston the air is forced out through the outlet valve, through an air pas sage outside the cylinder and through a passage in the pump mounting plate and the casting of the lower valve group.
The underside of the piston is served by a single air passage in the mounting plate connecting a central hole at the bottom of the cylinder to the lower valve group where the inlet and outlet valves are mounted one on top of the other with the inlet valve at the bottom. The air inlet to the inlet valve is in the underside of the valve group.
WAYCOCK AND OUTLET NOZZLES
7. The left hand cylinder outlet from the lower valve group passes direct to the left hand outlet nozzle to which the diver's air pipe is connected and this is the only outlet for the left-hand cylinder. The right hand cylinder outlet from its lower valve group is direct into the hollow centre of a waycock valve plug. The plug is operated by a waycock lever at the top of the pump through an operating rod fitted over the square head of the plug. The lever has two positions marked 'Two Divers' and 'One Diver-Deep Water' respectively. With the lever to 'Two Divers', a hole in the side of the plug valve aligns with an air passage to the right-hand side nozzle and diver's air pipe. In this position, each cylinder is supplying its own outlet nozzle.
With the cock lever to 'One Diver-Deep Water', a second hole in the cock plug aligns with a cross connecting pipe into the left-hand nozzle, while the first hole is blanked. Under this condition, both cylinders are delivering to the left hand nozzle.
8. A pressure gauge is connected to each outlet nozzle pipe. Both gauges are graduated from 0-100lb./sq.in. air pressure and from 1 to 226 ft sea water. These gauges indicate the diver's depth and the pressure at that depth.
9. To prevent loss of efficiency due to hot cylinders when the pump is delivering at high pressure, water cooling is provided for the cylinders and for the lower valve groups. It should not normally be required in cold weather or when diving to shallow depths.
An open top tank is fitted around the two cylinders, it is filled through a copper vessel in the back of the pump chest and overflows into a second tank fitted around the lower valve groups. The cylinder tank is drained through a drain plug at the back of the pump chest. The smaller tank overflows through a hole in the right side of the pump chest and is drained by a drain cock in the bottom of the tank. Care must always be taken that the overflows are clear, otherwise water may reach the piston rod gland.
Care must also be taken that water does not splash into the inlet valves.
10. In the lowest point of each air passage between the lower valve group and the outlet nozzle is fitted a hollow drain plug which acts as an oil catcher. They are accessible under the valve group cooling tank and are screwed into the drain socket onto a leather washer.
CALCULATION OF PUMP REVOLUTIONS FOR A GIVEN SUPPLY OF AIR
11. As stated in Part 2 of this Manual, the diver must receive at least 1·5 cu. ft of free air for each atmosphere of the absolute diver's pressure, therefore, at 2 atmospheres absolute (30 lb/sq. in.) for instance, the pump must deliver 2 x 1·5 = 3 cu. ft of air per minute at a depth of 33 ft. Each cylinder is designed to deliver 0·1 cu. ft of air per revolution of the pump handwheel; therefore, considering one cylinder only, it requires 30 revolutions per min. to supply the required 3 cu. ft of air.
There is a direct ratio of 1:10 between the quantity of air required and the diver's pressure, i.e. 1·5 cu. ft for one atmosphere (15 lb/sq. in.), 3 cu. ft for two atmospheres (30 lb/sq. in.), et seq. Each cylinder delivers 0·1 cu. ft for 1 revolution of the pump hand.wheel, which is also a direct ratio of 1:10. Hence:
(Absolute pressure in lb/sq. in.) / No. of cylinders in use = Revolutions required
The pump suffers a loss of output when delivering against a pressure, but this loss may be counteracted by multiplying the theoretical revolutions obtained by a Pump Factor (see para. 37) which represents the leak of the pump. This will give the practical revolutions actually required to maintain the diver's air supply.
Example: a diver's pressure at a depth of 44 ft is 20 lb/sq. in., therefore the absolute pressure is 20+15 = 35 lb/sq. in. The theoretical revolutions required are:
35 / 2 = 17·5
and if the pump factor is 1·18, then
17·5 x 1·18 = Practical revolutions = 20·65 (21)rev/min
USE OF THE PUMP
12. It is essential that the pump is very securely lashed down and that there is adequate room for the pumping party to operate.
When sending a diver down to a depth not exceeding 33 ft his air pipe is connected to the right hand nozzle and the waycock put to 'Two Divers', the left hand nozzle is then free to supply a standby diver as required. When diving in depths in excess of 33 ft the waycock is set to 'One Diver-Deep Water' and the diver's air pipe connected to the left hand nozzle which is now receiving the output of both cylinders. The stand-by diver will be supplied from a second pump in a similar condition.
13. The four-way junction or 'manifold' shown in Fig. 62.2 is used for connecting two or three pumps together. Three of its four connections are fitted with shut-off cocks, and are used for connecting the pumps; the fourth, which has no cock, takes the diver's air pipe. The pumps are connected to the junction piece by 45 ft lengths of air pipe which are always connected at the pump end to the left hand nozzle of the pump.
If one pump has to be disconnected while the diver is under water, the connection cock must be shut off before the pump is disconnected. If, on the other hand, a pump is to be connected while the diver is under water, it must be heaving around and be delivering air of equal pressure to that being delivered by the pump in use before the shut-off cock is opened.