Removing Waste Water
Some important factors to consider when dewatering a building
photo courtesy FDNY
DEWATERING a premises following a fire is a problem which may face a fire department at any given time. The need for this operation may also exist without an accompanying fire after a break in a water main or following a flood. Certain areas of this country which are regularly plagued with flood conditions have adopted standard procedures for dealing with this menace in order that community welfare be maintained. Reports of this work are usually recorded in the spring season along any of our great and not so great rivers.
At other times hurricanes may create emergency conditions in sections of the country not normally considered critical flood zones and cause great hardships which the fire service may be called upon to alleviate. For example, several years ago the Yardley, Pa., Fire Co. No. 1 logged over 61,000 man-hours of labor following a hurricane-created flood. In an effort to speed rehabilitation of the community, the company pumped out over 400 cellars of homes of unfortunate victims.
During this same period the Philadelphia Fire Deprtment was called upon to assist at than 90 dwellings and 50-odd business establishments which wee flooded following inundation of the city’s southwest area when a dike burst. An extension of the city subway system, then under construction, was also pumped dry in order that work schedules could be maintained.
Necessity for dewatering
The need for dewatering operations may be dictated by several considerations. It may be necessary to facilitate fire fighting safety, for salvage purposes or to promote sanitation and the safety of the general public. The methods employed are fairly standard throughout the fire service and in general, consist of the use of eductors, sometimes referred to as “siphons”; portable pumps; or in certain circumstances, the use of regular fire apparatus pumps.
Eductors are a popular and common device for performing dewatering operations. The equipment consists of a casing, open at the bottom, through which the water to be removed is drawn. A strainer, to prevent debris being drawn into the device, may be attached to the bottom end. A vertical tube or discharge pipe, centered in the casing, carries the water upward to the discharge. A fire department coupling is provided at the top for the connection of a hose line to lead the waste water up and out of the premises.
A nozzle is placed in the casing in such a manner that water admitted to it under pressure is forced upward as a jet in the discharge pipe. This nozzle is fitted with an exterior coupling permitting the attachment of a hose line from a pumper or hydrant which provides the water under pressure. As the water under pressure is forced through the nozzle, into the casing and out of the discharge, a venturi action occurs. A partial vacuum, or pressure differential, is created within the casing and the waste water is forced up through the base of the eductor by atmospheric pressure acting on liquid surface, and is discharged together with the water supplied to the device.
The velocity of the water being discharged from the nozzle forces the total amount of water being removed through the device upward, with the maximum depending upon the pressure at the nozzle. The efficiency of an eductor is low, measuring between approximately 30 and 60 per cent, but it will still move large quantities of water.
Chart 1—Waste Water Removed in Gallons per Minute
Fred Shepperd in “Fire Service Hydraulics” (page 154) devotes a section to the principles of operation and the hydraulics involved. Beyond this, very little study had been devoted to the unique devices until the New York Fire Department researched the various operating characteristics in connection with the design of a new eductor model. A report of these findings by Captain Thomas W. Ryan, Supervising Engineer Unit No. 1, published in WNYF, April 1957, forms the basis of much of the hydraulic performance tables used in this article.
The figures in Chart 1 indicate graphically the importance of keeping friction loss in eductor layouts to a minimum. While 2 1/2-inch hose was not employed on the discharge side, due to the friction factor, it is possible that results comparable to 3-inch hose may be obtained if two lines of 2 1/2inch hose are wyed from the device. Eductors are presently manufactured in the 2 1/2-inch size rated at 18,000 to 20,000 gph at a 12-foot lift, and also in 1 1/2-inch size which is rated at 4,000 to 6,000 gph at the same lift.
Tests have been made of the 2 1/2inch eductor mentioned above. In one case the device was supplied by a single 2 1/2-inch line at a 75-psi jet pressure. Discharge was through parallel 2 1/2-inch lines and lift was a measured 20 feet. The discharge was 524 gpm. The amount of water removed was 274 gpm, indicating an efficiency of about 52 per cent.
Tests made th_____ 1 1/2-inch eductor demonstrated th_____e e_____e_____ of increasing the lift. At a l0-f_____l_____ and connected to a hydrant with 100 psi, the small eductor removed 6,000 gallons per hour with an efficiency of approximately 59 per cent. At a 20-foot lift, the same eductor removed 5,200 gph, or an efficiency of 49 per cent.
The great value of the eductor is its ability to work at lifts which are well beyond the capabilities of pumps. For example, with a jet pressure of 175 psi and discharge lines of parallel 3-inch hose, the FDNY tests show that 136 gpm were educted at a 60-foot head. Other advantages immediately apparent are the absence of possible damage to pumpers due to pumping gritty or dirty water and the absence of the danger of noxious fumes being introduced into poorly ventilated subcellars, etc. Furthermore, the pump supplying the eductor may be some distance remote from the scene of operations, thus lessening or eliminating excessive noise in the immediate area.
Eductors may be employed directly from good hydrants where lifts are not too great, thereby removing entirely any objectionable noise. Results naturally will not be comparable to those where pumpers are employed to increase the water pressure.
The New York Fire Department offers the following suggestions for the use of eductors based upon their experience and data collected from their research:
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- Limit the length of supply and discharge lines as much as possible to lessen friction loss. At the same time provide sufficient hose in the original layout to permit later removal into the lowest or innermost area of the building.
- Use the largest diameter discharge hose possible.
- Avoid kinks in the line.
- Keep eductor intake well below the surface of the waste water, yet avoid placing it on the floor or bottom of the area until the operation is nearly completed.
- Friction loss in long discharge lines can be reduced by inserting a wye as close to the eductor as possible and attaching parallel lines.
- Where necessary due to limited working space, it may be expedient to connect a length of hard suction to the intake of the eductor. This makes it unnecessary to place the device itself in the water. When used in this manner, a strainer should be employed.
A foaming white discharge indicates inefficient operation or nonfunctioning. Check for the following conditions: (1) Air entering eductor; (2) head too great; (3) eductor clogged by debris; (4) supply or discharge lines too long, kinked or blocked; (5) insufficient engine pressure.
It is generally preferable to operate without a strainer which has a tendency to frequent clogging. Small debris will pass directly through the eductor and be carried away. A large wire basket, perforated ash can or other improvised device will keep large debris from reaching the intake. If such a device is not available, the strainer should be placed in position when the level of the water drops near the bottom. If the eductor clogs, it is usually not necessary to remove it from the water. Shut down the source of supply and permit the head of water in the supply and discharge lines to backflow and clear the stoppage.
Using portable pumps
A very popular method for removing unwanted water is by means of portable pumps. These lightweight units are readily adapted for this service as they are easily carried to a convenient location for performing the necessary task. The limitations of a portable pump must be recognized at all times. Suction lift and friction losses are important considerations as the small pumps are not designed for the same efficiency of operation that may be expected with larger apparatus pumps. Rather they are a compromise between size, weight and performance.
Most portables used in the fire service will deliver about 100 gpm under rated operating conditions and many will exceed 200 gpm for quickly removing large quantities of water. Maximum capacity may be realized at any practical lift if it is possible to discharge the water directly from the pump outlet. The addition of even one length of hose immediately reduces the output due to the head losses introduced.
The generation of hazardous exhaust fumes from the driving engine of a portable pump must be borne in mind at all times. These small units should be operated from the exterior with the suction hose lowered to the water to be removed. One type of portable pump on the market eliminates the problem of dangerous fumes, and at the same time, removes the necessity for hard suction hose. It is an electrically driven portable which may be entirely submersed in water for operation. The electrical power source may then be placed in any convenient location. A disadvantage of the presently available models is the limited capacity.
CHART 2—Table Showing Minimum Discharge Which Should be Expected of a Pumper in Good Condition Operating at Draft at Various Lifts.
CONDITIONS: Operating at Net Pump Pressure of 150 psi; Altitude of 1000 feet; Water Temperature of 60°F; Barometric Pressure of 28.94″ Hg (poor weather conditions).
Fire apparatus pumps
It has been a general rule of the service that fire apparatus not be employed for dewatering purposes. Being emergency apparatus, it is risky to subject them to such service where grit and other foreign matter may cause excessive wear or damage to the pump. However, there are extenuating circumstances where the large pumps are most efficiently employed. This condition may exist following a flood caused by a cloudburst, hurricane, etc., which affects an extended area. In order to quickly relieve the discomfort of the citizens and contribute to the health and general welfare, it may be necessary to use pumpers.
The same problem exists with apparatus pumps as with portables, although not to the same degree. Suction lift must be considered as well as discharge head. It is possible that volumes exceeding the normal ratings of the pump may be achieved by limiting the lengths of hose on the suction and discharge side. Since pressure is of no great importance, open hose butts or large deluge nozzles may be employed. In this manner, the greatest percentage of total work performed by the pump is devoted to “lifting” a large volume and the job time will be substantially reduced. Friction loss in the hard suction hose connected to the pump is definitely a consideration. The hose should be as large a diameter as is possible and only as long as is necessary to perform the task. Reference to Chart 2 will quickly indicate the performance which can be expected of a pumper in good operating condition.