CHAPTER # 5
MAINTENANCE
5.1 MAINTENANCE OF SEWERS
The principal efforts in the maintenance of sewers is to keep them clean and unobstructed. A sewer system, although buried, cannot be forgotten, as it will not take care of itself. It will corrode, erode, clog, or otherwise deteriorate. The capital investment in a sewer system financially justifies preventive maintenance. Work to be done in the maintenance of sewerage works, particularly the sewer system, includes inspection, measurements of rates of flow, cleaning, flushing, repairs, supervision of connections, protection of existing sewers, prevention of explosions, valuation and other duties.
5.1.1 OCCUPATIONAL HAZARDS
Hazards in the maintenance of sewage works include physical injuries, infections, gas poisoning and asphyxiation.
Gases may be met in sewers, manholes, wet wells digestion tanks and other poorly ventilated enclosures exposed to the sources. The depletion of the oxygen as a result of organic processes in closed or poorly ventilated chambers may leave insufficient oxygen for the support of human life. The absence of oxygen is as dangerous a hazard as is the presence of toxic and asphyxiating gases.
Employees in sewer-maintenance work and in the operation of sewage-treatment plants are more than ordinarily exposed to personal injuries. Slippery ladders, walks and steps; heavy lifting; fast-flowing water and deep tanks to fall into; exposed machinery, electric current; and explosive and asphyxiating gases are ever
present and lead to the necessity for the exercise of every reasonable precaution, Guard rails, natural and artificial illumination, ventilation and safety first measures should be generously provided. First aid kits and gas masks should be available, and all personnel should lie instructed in their use, and in methods of resuscitation from drowning, electric shock and asphyxiation.
5.1.2 INSPECTION
Inspection of sewers is made to reveal clogging; to observe the condition of control mechanisms, and to service them; to examine the condition of the structure, to measure the rate of flow in the sewer; and for other purposes. Attention should be given regularly to those sewers that are known to give trouble, whereas less troublesome sewers may be given less attention Inspection and flow measurements may be necessary during storms to determine the relation between maximum flow and sewer capacity
The routine, inspection of sewers too small to enter is made by observations from manholes, It is not always necessary to enter the manhole to make required observations, if the sewage is flowing as freely at one manhole as it is at an adjacent manhole it may be assumed that the sewer between manholes is not clogged. No further inspection may be necessary unless there is reason to suspect some other difficulty in the sewer between manholes. If the sewage is backed up in a manhole it indicates that there is an obstruction in the sewer. If the sewage in a manhole is flowing sluggishly and scum is present, it is an indication of clogging, slow velocity or septic action in the sewer. Sludge accumulations on the sloping bottom of the manhole or signs of sewage having been high up on the wall indicate at occasional flooding of the sewer due to inadequate capacity or to clogging.
Sewers too small to enter are cleaned by thrusting rods into them or by dragging some from of instrument through them.
Flexible rods or steel tapes are widely used for rotting sewers too small to enter. 'The steel "tap" consists of flexible rod, about 1/8 in. thick and 1 to 2 in. wide.
The rod, to the advancing end of which, a cleaning tool may be attached, is pushed into the sewer. It may be vigorously twisted at the same time, by a gasoline or electric motor, revolving at a speed of 750 rpm or more . This device is advantageous in a crooked sewer.
Obstruction may be dislodged by shoving a fire hose down the sewer, the; hose discharging water at high pressure through a 1" or smaller nozzle. The water pressure stiffens the hose and, together with the support from the sides of the sewer, makes it possible to push the hose 100 ft. or more into the sewer.
A self- propelling nozzle attached to the hose is also used.
Sewers are seldom so clogged that no channel whatever remains. As the clogging in a sewer progresses the passage becomes smaller, thereby increasing the velocity of flow around the obstruction and maintaining a passageway by erosion. This phenomenon has been taken advantage of in the cleaning of sewers by the use of "pills". These consist of a series of light spheres of different diameter. One of the smaller balls is dropped into the sewer through a manhole above the obstruction. When ball strikes tile obstruction it is jammed against the top of the sewer. Sewage is backed up and seeks an outlet around the ball, thus clearing a channel and washing the ball along with it. The ball is caught at the next manhole below.
The feeding of balls into the sewer is continued, with larger and larger sizes, until the sewer is clean. The method is particularly useful for the removal of sludge deposits but it is not effective against roots and grease.
Fungus growths sometimes found in sewers are a result of favorable environment and suitable food supply. Successful removal may be accomplished by changing conditions in the sewer through depleting the oxygen present, changing the chemical reaction of the sewage (acidity or alkalinity), or adding a fungicide such as copper sulfate. The oxygen content of the sewage may be changed by blocking the sewer to induce septic conditions and lime may be added to increase alkalinity.
5.1.4 FLUSHING SEWERS
Sewers can sometimes be cleaned or kept clean by flushing. The desired results are obtained most effectively by a sudden rush of deep water down the sewer at high velocity. Depth and velocity together are necessary; either alone will be ineffective. A slow, steady flow will have little beneficial effect.
The most effective flush is obtained by a volume of water traveling at a high velocity and producing an hydraulic bore.
Flushing may be automatic and frequent, or hand flushing may be resorted to at intervals. Automatic flush tanks, flushing manholes, a fire hose, a connection to a water main, a temporary fixed dam, a moving dam, and other methods are used in flushing sewers.
The effectiveness of flushing may be increased if the sewer is inspected and large clogging objects are removed before flushing is attempted.
After laying the sewer pipes or sewerage system the proper maintenance of the system is required for its effective operation
For maintenance the following factors may be included
1. There should be sufficient number of maintenance staff i.e., sewer man, pump operators etc.
2. The whole sewerage systems should be checked regularly by the staff workers. They should check for sludge deposition inside the sewer and the manhole.
5. Cracking of sewer piping may also be noted occasions. It particularly happens when the sewer is laid at depth, closer to the ground level, due to load or wearing and the sewer breaks. Such sewers should be removed immediately.
6. A single method adopted for cleaning of sewers is that a sewer man enters into the manhole with the help of small stair steps provided in the manhole and try to push the accumulated sludge in the sewer to the next manhole with the help of long stick or rod. When the sludge reaches on next manhole it is taken out of it. This method is quite effective and the chocked sewer should be cleaned properly using this technique-
7. The man at disposal tank (pump station) should operate the pump after suitable regular intervals. So that the sewage present in the tank is pumped out and the tank is able to take the coming sewage.
8. The disposal tank should be checked and whenever tile sludge is accumulated up to the height of suction valve, it should be taken out. Other wise the valve will be blocked, suction will be stopped, tank will over flow and the system will be chocked.
5.2 MAINTENANCE OF PUMPING STATIONS
Preventative maintenance is always preferable to emergency repairs: routine inspections should include the impellers and balancing discs; the bearings; and the general alignment of the shafting. In the split casing type of centrifugal pump, the whole of the rotating elements of the pump are exposed by lifting off the cover.
Failure of pumping plant, particularly sewage pumps, may not necessarily be due to any mechanical fault-frequently the trouble will be due to a pump or pipeline blockage (rags, etc.); occasionally it may be loss prime due to ingress of air.
5.2.1 PRIMING
Efficient priming of a pump and the suction spaces is essential. since an impeller working in air will not draw water. Air relief valves on the top of pump casing (if fitted) must function correctly and in an automatic station, the levels of the pump cut-in and cut-out must be such that the pump is always primed ready for operation. Where foot-valves are flitted these must be checked to that there is no possibility of a pump running ‘dry'. When there is loss of prime, it may be due to leaks at pipe joints, valves, glands etc.
5.2.2 BEARINGS AND BALANCE DISCS
Small pumps may have ball or roller bearings, but larger pumps will probably have thrust bearings, either oil or water lubricated. In a high pressure multi-stage pump a hydraulic balance disc may be fitted to take part of the end thrust; this balance disc or its facing may wear sufficiently to allow the impellers to get out of line. Wear on the disc will be minimized by ensuring that little or no sand or silt is taken up by the pump.
The bearing of all pumps, motors and similar rotating machinery must turn freely and must be kept well lubricated with grease or oil. Oil is preferable where the temperature or speed of rotation is high, but otherwise grease is normal. Care must be taken not to over-lubricate the bearings of electric motors.
5.2.3 PUMP GLANDS
When packed glands are used these will require regular maintenance and the packing must be inspected and renewed from time to time.
A faulty gland results in dirty conditions in a sewage pumping installation and it can also cause excessive wear at the shaft and the shaft sleeve. Some slight leakage is normally allowed, to provide cooing for the packing gland and to ensure that the gland is not so tight that rapid wear will occur; this leakage should be about one drop per second for small pumps, and greater for larger units. The wear will normally be taken up by a shaft sleeve to avoid wear on the shaft itself. Where mechanical seals are used in lieu of packing, the manufacturer's instructions should be carefully adhered to as regards maintenance.
If the pump is working under a positive suction head, the suction gland must either have a mechanical seal or it must have a water-sealed lantern ring adjacent to the pump impeller so that air will not be drawn into the gland, and also to avoid the possibility of gritty water being forced through the stuffing box.
5.2.4 PUMP IMPELLERS
Sewage pumps are liable to blockage between the impeller and the pump casing as rags and other fibrous matter tend to become wrapped around the impeller. If the pump is not of the split-casing type, handholds should be fitted to give access to the impeller so that blockage can be cleared.
Impellers eventually become worn and must be replaced, as the efficiency of the pump decreases as the clearance between the impeller and the valute casing increases beyond the design clearance. Wear will be more rapid in pumps dealing with sewage or other water which may contain appreciable quantities of grit. It is generally more economical to fit a new impeller than to recondition the old one. Many sewage pumps incorporate 'wear ring' which must be replaced from time to time; these help to minimize the wear on the impeller.
5.2.5 ELECTRIC MOTORS AND SWITCHGEAR
Electric motors and other electrical equipment generally require very little maintenance, provided they are maintained in clean condition, free from oil, dust, moisture etc. The bearings of motors must be lubricated to prevent them from running hot but, as stressed above, care must be taken not to over-lubricate, otherwise oil or grease may get into the electrical connections.
Should a motor run at a speed much less than intended, there may be a break or loose electrical connection, particularly in the rotor lead. Trouble may also be caused by dirty slip rings (in that type of motor) giving poor brush contact; this will be apparent from sparking and flashing and any worn brushes should be replaced.
5.2.6 VALVES AND PIPELINES
Once installed, pipes and valves should require very little attention. At many pumping stations the sluice valve and penstock will remain permanently open (or closed) and it is a wise precaution to operate their gates fully a few times once every six months or so. The spindles should, of course, be kept lubricated with grease.
The suction pipes must be checked from time to time to ensure that all joints arc air-tight as any ingress of air will affect the performance of the pump. when strainers are fitted, there must be kept clear of debris and the foot valves must be examined occasionally to ensure that the flap is clear. Suction pipes must be installed so that their lower ends are immersed at all times and so that there are no "humps" or "dips" between the sump and the pump. Where the sump is shallow, the suction pipe should be checked to ensure that it is adequately submerged, otherwise vortices may form, allowing to be drawn into the pump.
The reflex valves on pump deliveries must be checked to ensure easy operation. Trouble is more likely with sewage pumps than with other types of installation as there is then more likelihood of the valves being jammed with silt and rags. A reflex valve which closes too slowly due to silting will be liable to "slam" due to pressure surges in the pipeline.
Pressure surges in the pipeline. For sewage pumping, it is wise to install valves with their spindle hearings or trunnions outside the valve casing, many engineers specify that reflux valves in sewage pumping installation shall have external operating levers so that they can he opened manually to clear any solids. External levers on the reflux valves also allow the pumps and suction pipes to be back-flushed from the rising main, a weight can also be attached to the lever to ensure that the valve closes quickly.
Sluice valves should generally be fully opened at all times when a pump is running. At an automatic station this should be checked occasionally.
On no account should closing a valve on the suction side regulate a pump, as this will encourage cavitations.
5.2.7 THE WET WELL OR PUMP SUMP
Sewage pumps will draw from a wet well, which may include screens or comminutors to protect the pumps. Screens are. generally not suitable for small, isolated stations. The wet well must be inspected regularly, particularly after a storm, and all grit and debris must be removed. Screenings must not be allowed to accumulate at the station, as these will cause nuisance from odour.
5.2.8 OPERATION AND MAINTENANCE
Regular inspection and preventive ensure continued, reliable operation of the entire submersible pumping system. All stations, pumps and operating equipment should be inspected at least once a year, and more frequently under severe operating conditions. One of the major advantages of a submersible station is the ability of the service technique to handle most maintenance and service on site, without entering the wet well. All equipment in the station should be backed by manufacturers service manuals. This material should be carefully read, filled and should be consulted whenever servicing is required.
To minimize the risk of accidents in connection with service work, the following rules as well as all applicable laws, regulations and manufacturers must be followed. Note and read all safety precautions before performing any operation or maintenance procedure.
· Be aware of the risk of electrical accidents.
· Make sure that all lifting equipment, when used, is in good condition.
· Provide a suitable barrier around the work area, e.g, a guard rail.
· Make sure that all personnel have a clear path of retreat.
· Use safety helmets and protective shoes or boots.
· All personnel working with sewage system must be vaccinated against any disease that can occur.
· Before entering the wet well make sure there is sufficient oxygen and that there are no poisonous gases present.
Since sewage pumps are designed for use in liquids which can be hazardous
to the health, make sure that all equipment has been thoroughly cleaned.