Chemical dispensing equipment, as defined in ANSI/CAN/ASSE/IAPMO 1055-2020, Performance Requirements for Chemical Dispensers with Integral Backflow Protection, consists of several independently operating components with different functions. The dispenser contains a shutoff valve to control flow through the device. In addition, there is the eductor/mixing chamber and an integral backflow prevention device. Both are crucial to the equipment’s operation.
The eductor/mixing chamber uses a water eductor, which is based on Bernoulli’s principle. This principle states that when the velocity of a fluid is increased, the pressure will proportionally decrease. The reverse is also true. So how does this work?
As the working fluid – in this case potable water – flows through the device, it encounters an orifice, or nozzle, creating a restriction. This restriction reduces the cross-sectional area of the flow path, thereby causing the velocity of the fluid to increase through the restriction as it enters the mixing chamber. As Bernoulli’s principle predicts, the pressure in the mixing chamber decreases, corresponding to the velocity increase.
In fact, this pressure change is such that it creates a vacuum. Included in the mixing chamber is a connection to the chemical concentrate which is to be mixed. This chemical-feed connection is located at the middle of the restriction. The vacuum creates a negative pressure, allowing atmospheric pressure to push the chemical into the device through this connection to create the proper mixture of chemicals and water in the dispenser’s mixing chamber. This process is known as the Venturi effect.
By varying the size and shape of the restriction (i.e., controlling velocity) and the feed connection’s cross-sectional area, it is possible to engineer a chemical dispenser with a very specific proportional mix of chemical to water. As the mixture leaves the mixing chamber, the exit area increases, the velocity drops, and the pressure increases. This aids in the mixing, allowing the pressure to smoothly increase to the external pressure downstream.
Since the chemical dispenser is using a potable water supply and is introducing chemicals to the discharge of the device, it is important to protect the water supply from cross contamination. Hence, the need for the backflow preventer component of the device. Without backflow protection, a situation that would move the chemical into the supply source could occur. For example, an event in the supply system could occur, which would result in the supply pressure being lower than the atmospheric pressure, creating a negative pressure situation. In this case, if there is no backflow protection, the concentrated chemical mixture would be drawn back into the potable water supply.
The adopted plumbing code and product standards specify what type of backflow protection is required for chemical dispensing equipment. Listed ASSE 1055 chemical dispensing equipment has the backflow prevention device located within the dispenser. One type of acceptable protection would be the use of an air gap. An air gap provides protection by flowing the dispenser’s water supply through an atmospheric opening, thereby making it impossible for backsiphonage to occur.
Another type of backflow protection used in chemical dispensers is an elastomeric gap with an air inlet. These operate like an atmospheric vacuum breaker. When water is flowing and the device is under pressure, the elastomeric gap expands and air vents are closed. When there is no pressure, the elastomeric gap returns to its original size and the air vents are open to atmosphere. The vents are required to be designed with a bias to a normally open position. Under atmospheric or negative pressure, the vents are open, which breaks the vacuum between the supply and the chemicals.
Depending on the dispenser design, you may also find ASSE 1055 listed chemical dispensing equipment protected with an ASSE 1001 listed atmospheric vacuum breaker as the equipment’s integral protection.
Types of backflow protection that do not meet the requirements of the ASSE 1055 and do not contain integral backflow protection, but are sometimes found on the piping supplying potable water to chemical dispensers, are the pressure vacuum breaker assembly (ASSE 1020), the spill resistant vacuum breaker assembly (ASSE 1056), and the reduced pressure principal backflow prevention assembly (ASSE 1013).
Chemical dispensing equipment design helps provide a safe means to furnish a correctly mixed chemical solution for the intended end use. For a small device, it has a very important role in protecting public health and safety. Amazingly, this small device is packed with science and engineering principles to achieve this purpose.
Article by Terry Burger first appeared in ASSE’s Working Pressure magazine