Types Of Chemical Delivery
Getting the cleaning chemical to the surface to be cleaned requires the chemical solution either be sprayed on the surface separately or delivered along with the water spray from the cleaning system. Most high-pressure cleaners allow for automatic delivery of chemicals through the system. This blending of a measured amount of cleaning chemicals or detergents into the flow of water is necessary for a fully integrated cleaning system. The chemical delivery system allows the high pressure cleaner to be used as a chemical applicator. The operator does not have to manually dilute chemical solutions since flow can be precisely controlled.
Upstream Or Downstream
Chemical supply systems fall into two major categories. If chemical enters the system before the pump or on the low-pressure side of the pump it is called upstream or low-side injection. Chemicals delivered upstream pass through the pump. If chemicals are delivered to the system on the high-pressure side of the pump the process is called downstream or high-side injection. Chemicals injected downstream do not pass through the pump. Upstream injection allows delivery of chemical at high pressure. Downstream injection allows the use of chemicals, which should not be run through a high-pressure pump.
Upstream Chemical Delivery
Upstream chemical delivery relies on the pressure differential between the inlet line and the chemical container to draw chemicals into the fluid stream. Although it is generally said that chemicals are sucked into the system, what actually happens is that chemicals are pushed into the system when pressure at the inlet side is reduced below atmospheric pressure by pump action. Upstream delivery allows application of chemicals under either high pressure or low pressure.
Most downstream injection systems use a venture injector, which also relies on a pressure differential to draw chemical solution into the system. Downstream injection generally requires that chemicals be applied at a pressure lower than normal operating pressure.
Multiple Chemical Delivery Systems
Some cleaning systems use more than one chemical solution, the two-step truck washing systems use an acid cleaning solution and an alkaline to neutralize the acid. The chemical delivery control systems on these cleaners may be a bit more complex but the basic process of delivering chemicals is the same. Other systems may use a pulse pump or other means of introducing chemical into the cleaning system.
Some chemical valves, such as the ST66, allow for a choice between two different types of chemicals. The ST66 two-point valve is simply a regulating valve, which allows metered flow of chemical through one or the other of two inlets. Generally, in the case of failure, this valve type should be replaced rather than repaired or rebuilt.
Pumping Chemicals Into The System
Chemicals may also be forced into the water flow with a small pump. This pump may have its own small electric motor or it may be driven in another manner. One method involves using the pulsation of the main pump to drive a sort of piggybacked pump known as a pulse pump. Use of a pump to force chemicals into the system generally allows the chemicals to be supplied either upstream or downstream.
Some equipment designs use a pump to introduce water softening or scale inhibiting chemicals into the system while using an injector to introduce cleaning chemicals. Water softening increases chemical effectiveness and lengthens equipment life by reducing scale buildup throughout the system.
The Chemical Delivery System
Most chemical delivery systems, whether upstream or downstream, have several similar or analogous components. These include the chemical container, chemical foot or foot valve, and the metering valve or injector. An upstream chemical delivery system generally requires that the cleaning system be fitted with a float tank. A downstream chemical delivery system requires some form of pressure adjustment in the system to reduce system pressure to the point where chemical may be delivered. Usually a cleaning system with downstream injection will be equipped with a variable pressure lance or a nozzle with pressure adjustment capability.
The Chemical Container
Cleaning chemical can be dispensed from almost any type of container in which the chemical line may be submerged. In many cases, with stationary systems, the chemical line may simply be dropped into the 5-gallon drum in which the chemical was delivered. Five gallon buckets and one-gallon bottles can also be used as chemical containers. Most full-featured high pressure cleaning systems come with a special chemical container, often fitted with a cap with an opening to allow entry of the chemical line while preventing the entry of debris into the chemical container.
Almost any container, which will hold the chemical product and not react with it, can be used as a chemical container. (Using an aluminum container to dispense acid is not recommended since the acid would attack the aluminum container.) Any chemical container other than the container in which the chemical is originally packed should be checked before use to make sure it is empty and free of contaminants and debris. It is also a good idea to use a chemical container that is covered to the extent that entry of debris into the container is prevented. Debris in the chemical delivery system can cause a number of fluid system problems including clogged pump check valves in upstream delivery systems and flow unloader cycling in downstream delivery systems.
The Chemical Foot Or Foot Valve
Most cleaning systems are equipped with a strainer or combination strainer and check valve on the inlet end of the chemical line. This is called the chemical foot or chemical foot valve. This foot is generally cylindrical with a hose barb on one end. The chemical foot may be made of plastic with perforations to strain large debris from the chemical or it may be a simple plastic or metal cylindrical framework designed to hold a replaceable or reusable mesh screen as a strainer. The chemical foot may be weighted as well to keep it from floating to the top of the surface of the chemical supply.
The chemical foot may also be fitted with a check valve to prevent water from the system from siphoning back into the chemical container. Siphoning can both dilute the chemical to a point where it is no longer effective and give the appearance that no chemical is being used because the chemical container never empties. A chemical foot with a check valve is generally referred to as a foot valve. Since many downstream injectors are equipped with check valves, it is not so important to use a foot valve with a downstream delivery system as it is with an upstream system where there may be nothing to prevent backflow from the pump to the chemical container except for the chemical metering valve. The chemical foot should be inspected regularly for clogging, loss or damage and cleaned, replaced or repaired as necessary.
The Chemical Line
Low-pressure tubing or hose is used to transport the chemical from the chemical container to the metering valve or injector. Often a transparent or semi-transparent material such as polybraid or plastic tubing is used for the chemical line. Most chemical lines are ¼ ID (interior diameter) tubing. The chemical line slips onto a hose barb on the valve or injector and onto a similar barb on the chemical foot or foot valve. A hose clamp should be used at the valve or injector connection to make certain the line is secure and to prevent air leaks. Air leaks at chemical line connections can contribute to a variety of fluid system problems. A flexible chemical line may also run from the chemical metering valve to the pump unless the valve is mounted on the pump or is hard plumbed to the pump head.
If polybraid is used for chemical line, it may be a good idea to purchase this tubing in lengths of 50 feet or more both due to lower cost and its usefulness in other applications such as emergency replacement fuel line in oil-fired burner systems. For proper chemical delivery the chemical line must be free of restrictions such as caused by clogging or kinking. Transparent tubing or polybraid can be visually checked for clogging. Non-transparent line can be checked for clogging by blowing through them or running water at public utility water supply pressure through the line. The chemical line should be inspected for kinks, twisting or binding against the equipment frame or other surface. Wire ties may be used to secure the chemical line so that it will not kink or bind. If the Chemical line has been binding or rubbing against a sharp surface, it should be inspected for abrasion and replaced if necessary.
Air must first be purged from the chemical line before chemical can be drawn into it. This usually involves nothing more than submerging the chemical foot in the chemical container, opening the chemical valve and allowing pump or injector action to draw air from the line. If the line is transparent, you can observe when liquid replaces air in the line. If the line is not transparent, suds in the system output are a sure sign chemical is being delivered.
While air is being purged from the chemical line in an upstream injection system, some pressure loss, irregular water flow or pump chattering may be noted. This is the natural result of air being drawn through the pump. These pump problems should cease once a steady flow of chemical is moving through the chemical line. A chemical line may be too long to draw chemical properly. Differences in pump capabilities and system design make it impossible to set an exact maximum length for a chemical line. However, if the line is longer than six to eight feet and the system will not deliver chemical properly, a simple test can determine if excessive chemical line length is affecting chemical delivery. Simply cut a shorter length of tubing, about three to six feet long if practicable, and run it to a chemical container. If the system now draws chemical properly, the original chemical line was too long. You may experiment until you determine the best chemical line length for your system, although in most cases it is best to use the shortest line practicable in a particular working environment.
The Float Tank
A float tank may be used for upstream or high-pressure chemical injection. The float tank provides a standing water source and allows chemical to be drawn into the system by the pump. If water is fed directly to the pump under standard municipal water supply pressure of 40 to 60 psi, the pump will not draw chemical. The tank breads down inlet pressure and allows chemical injection in the line between the float tank and pump. Some manufacturers refer to the float tank as the "break" tank for this reason.
Controlling Chemical Flow
A metering valve is used to adjust the volume of chemical flow in upstream chemical delivery systems, that is, where chemical is drawn into the system upstream or the pump between the float tank and pump inlet. The metering valve usually provides for infinitely variable adjustment from closed to wide open. The volume of chemical allowed into the system by the valve mixes with the volume. This allows the use of concentrated chemical without premixing. Usually the metering valve is an adjustable orifice that controls the volume of chemical entering the water flow. The smaller the orifice, the less chemical gets into the water flow. The valve generally consists of a body with hose barbs on each end and an adjusting assembly, which consists of the knob and stem or handle, which ends in the valve needle.
Orifice size and consequently the amount of chemical allowed into the system is controlled by the position of the valve needle. When the valve is completely closed, the valve needle should be firmly seated, allowing no chemical flow. The closer the needle is to the seat, the less chemical flow is allowed in the system. The chemical valve may be mounted so that the adjusting handle is part of the control panel. This allows for easy operator control of the amount of chemical delivered. Generally, when wide open, the metering valve allows for enough chemicals to enter the system to make a 5% to 7% chemical solution at the system outlet. A much more dilute solution of a good concentrated chemical is generally adequate, usually in the 1% to 2% range.
Measuring Chemical Flow
The amount of chemical being drawn into the system can be measured using a simple flow meter. Such meters are generally no more than a clear plastic block with a cylindrical chamber. A ball floats in the chamber so that the greater the flow, the higher it floats. The front of the meter is usually marked in graduations indicating gallons per hour of flow. The meter is equipped with hose barbs for mounting in the chemical line between the chemical container and the injector or metering valve. The meter should be mounted vertically and level for the most accurate measurement of chemical flow.
The flow meter can be handy in determining how effective a particular chemical product is since an exact measurement can be made of the amount of chemical needed to produce desired results in a particular cleaning application. The flow meter reading in gallons per hour can also be used to determine the percentage of dilution of the chemical product if the rated flow of the system in gpm is known. Simply multiply the system flow in gpm by 60 to convert it to gph, then divide this into the chemical flow as measured in gph by the flow meter. A 4 gpm cleaner using 2 gph of a particular chemical would be diluting the product to a solution of less than 1%, 0.83% to be exact. (4 gpm x 60 = 240 gph; 2 gph = .008333 or approximately 0.83%).
The Venturi Injector
Downstream chemical delivery systems generally employ a venturi injector, which uses a flow created vacuum, or more correctly, pressure differential, to deliver chemical solution to the system.
Venturi Injector Design
Physically, the injector is very simple in construction and operation with no moving parts except for the check valve ball and spring and the adjusting needle or screw. The injector consists of a metal body with threaded fittings for water input and output lines and a hose barb for the chemical line. A nozzle bushing or orifice, sized to accommodate the amount of flow through the injector, is set in the inlet line. Water flow is directed straight through the nozzle bushing orifice and injector body from inlet to outlet.
The hose barb, which houses a check valve ball and spring, is set at a 90-degree angle to the water flow. The check valve prevents high-pressure backflow into the chemical line. A needle valve may be set in the hose barb to allow variable adjustment of chemical flow. In valves of this design turning an adjusting ring surrounding the hose barb makes the chemical delivery volume adjustment. Turning the ring moves the valve needle closer to or further from the seat allowing for the reduction or increase of chemical flow through the system.
Another common design uses an adjusting screw set in the side of the hose barb housing. When tightened all the way down this screw will restrict or cut off the supply of chemical through the hose barb. Loosening the screw will increase the supply of chemical.
Venturi Injector Operation
The venturi injector is central to most downstream chemical delivery systems. Essentially the venturi injector is a jet pump – a sort of nozzle within the water line. Its operation is totally dependent upon water flow. Its operating power is provided by this flow. The power expended to pull chemical into the system is reflected in a pressure loss across the injector. The venturi injector gets its operating energy from water flow. This power expenditure is reflected in a pressure loss across the injector. That is, system pressure is higher before water enters the injector and lower after it leaves the injector. As liquid flows through the nozzle or orifice in the venturi injector, it accelerates into a jet as the water moves more rapidly to pass through the orifice at the same flow rate at which it moved through the water line to the injector. When it leaves the orifice, it enters another chamber where the narrow stream of water creates a pressure differential.
Pressure in part of this chamber and the chemical line is lowered to a level below atmospheric pressure by the road movement of water from the orifice through the chamber. This reduction in pressure in the chemical line actually allows atmospheric pressure to push chemical up the line into the system. After flowing through the nozzle bushing and chamber, water now mixed with chemicals, enters a passage or venturi where it loses velocity and pressure is recovered.
A portion of the flowing water’s energy has been imparted to the injected fluid so the reconverted pressure cannot be as high as the supply pressure. After the introduction of chemical into the stream at sub atmospheric pressure conditions, the injector then raises the pressure of the mixture by means of a diverging flow section, which slows down the fluid velocity. The outlet pressure will be less than the inlet pressure because of the energy transferred to the inducted fluid plus the frictional turbulence losses encountered in the abrupt acceleration and then deceleration of the motive fluid.
The pressure loss across an injector when operating normally is approximately one-third of the inlet pressure. Downstream venturi injectors generally require a reduction of system pressure to draw chemical. This is the result of the check valve’s action. At normal operating pressure, the check valve is held tightly shut and chemical cannot be drawn into the system.
Venturi Injector Orifice Sizing
The proper orifice size for system flow must be installed in a downstream or venturi injector for it to work properly. A smaller injector orifice than required for the system may cause a higher than normal operating pressure due to the incorrect injector orifice size. This mis-sizing can result in increased motor amp draw and increased starting load or power demand as well as reduced cleaning ability caused by the smaller orifice’s reduction of output flow.
If an injector is too large for the application the theoretical pressure loss will be less but there will be an insufficient pressure differential; the injector will not function and no chemical will be delivered. Injector selection is based on water flow and pressure. Sometimes these factors can be measured directly but generally are based on spray nozzle ratings and pump capacities. The selection can be complicated by pressure losses due to friction in hoses, pipes, and fittings, which must be added in.
Use of a two-stage injector can reduce the pressure loss that results with a single stage injector. In this design a small portion of the flow is diverted into a booster stage, a second, smaller nozzle and venturi combination, which, since it uses only a small portion of the flow to create the vacuum that sucks up chemicals, minimizes pressure loss. The two-stage injector is also referred to as a high-pressure injector since it allows for application of chemicals at close to normal operating pressure.
Besides proper sizing, chemical compatibility is another factor, which must be considered in selecting an injector. There can be problems with stronger chemicals such as hydrofluoric acid or aluminum brightener, and care should be taken to insure that the orifices, especially in two stage injectors, neither erode nor clog. Injectors with stainless steel parts are a good choice when using caustic or corrosive chemicals.
Chemical Delivery Problems And Service
The chemical delivery system is quite simple, but a minor malfunction in this system may produce symptoms, which may be diagnosed as more serious pump problems.
Air Entering The Chemical Delivery System
For chemical to be delivered, the chemical line must be submerged in the chemical container. If the suction hose is not submerged, then air is introduced into the system. If chemical is injected upstream of the pump, the introduction of air into the high-pressure pump can cause pump malfunction. If chemical is not being delivered, the chemical valve must be closed or the cleaner will not function properly. A substantial pressure loss will result. The operator is likely to call for service complaining that the cleaner will not develop adequate pressure. In this case service will involve nothing more than closing a chemical valve to restore proper equipment operation. It may be a good idea to ask the operator to check to see if the chemical valve is closed if the reported pump problem involves chattering, cavitation or low pressure.
Metering Valve Failure
The chemical metering itself may fail due to a number of wear-related reasons. The seat can wear or become pitted or the valve may wear to the point where leaking occurs. If there is no chemical strainer, debris may lodge in the valve. Use of harsh chemicals is generally the cause of excessive valve wear. Chemical scale may form in the valve so that the valve is stopped up or will not seat correctly. In most cases valve replacement is less expensive than rebuilding the valve. However, the valve may be cleaned of chemical scale in the field with a combination of soaking in descale solution and scraping out the valve seat.
Note: If there is scale in the chemical metering valve, there is likely to be scale elsewhere in the system. However, the scale will be chemical scale, which is easy to remove, rather than the much harder to remove scale which precipitates from hard water.
Metering Valve Disassembly
To access the valve needle and seat, loosen the nut in which the adjusting handle is mounted until the nut moves freely on the handle. The handle may be removed by simply pulling it off of the shaft. Remove the large, hex-shaped retaining nut from the chemical valve. Place the adjusting knob back on the shaft. Then back off the adjusting knob until the threaded needle is free of the threads inside the valve body. The entire adjusting assembly can then be pulled from the valve body by hand.
Once the adjusting assembly, including the handle, needle and knob, is removed, the needle and seat may be inspected for wear, clogging or scale. The gasket, which rides on the handle between the needle and knob, may also leak although such leakage is rare. Leaking may also occur at the inlet hose barbs. Make sure that the chemical lines are properly seated on the barbs. Any inlet or vacuum line should also be secured by a worm-drive clamp or equivalent hose clamp. If leaking continues, the barbs may be removed and reinstalled with new Teflon tape on the threaded male hose barb fittings.
Troubleshooting Chemical Delivery
If the unit has a chemical metering valve to control chemical flow drawn from upstream of the pump and the unit will not draw chemical, the first thing to do is to make sure the chemical valve is open. Start the cleaner with the chemical metering valve open. (You may need to turn it all the way in both directions to make absolutely sure it is open.) If the cleaner now draws chemical, adjust the valve to deliver the proper amount of chemical for the application.
Note: Check the control panel to make sure the "off" and "on" position markings are correctly oriented.
If no chemical is drawn when the cleaner is started with the valve open, the problem is likely to be in one of two areas: Either there is an air leak in the upstream side of the system which prevents the pump from drawing chemical into the system or there is a blockage or other factor preventing the system from drawing chemical.
Is the pumping unit vibrating or chattering?
If there is an air leak, the unit will be vibrating and/or chattering as a result of cavitation from air being drawn into the pump. Check the upstream side of the system for air leaks.
Is the chemical valve clogged?
Is the chemical line blocked or restricted?
Is the foot valve stuck or the strainer clogged?
If there is no vibration in the system, either the metering valve or chemical line may be clogged or the chemical foot valve and/or strainer may be stuck or clogged. Check for kinking or other restriction in the chemical line as well.