With any type of evolution, whether by nature or a technical evolution by man, there are certain driving forces that actually “get it going.” In our case, the need for safe water is one of the driving forces. Another driving force is an awareness of a problem – the relationship between cross-connections and illness. We also need to know that there is a solution; something that is practical that we can do something about. Specifically, backflow preventers have evolved because the way we use water evolves dramatically, from gravity systems to pressurized systems. Another reason is timing! Timing has to be right for things to happen. Timing of The Safe Drinking Water Act was one the key things that was a turning point for the water industry. Finally, and probably the most important part for me because I’m a design engineer, is technology changes in manufacturing and what it has allowed us to do in the design of backflow preventers.
I’m going to start with the 70’s because the 70’s is actually when I started in backflow protection and it was an important turning point in the water industry. We will start with backflow preventers yesterday and how they arrived at the point where they are now and discuss backflow preventers in general and end at focusing on the highest hazards and the “RP’s”. Things presented will relate to whether it’s a double check or an “RP”, a vacuum breaker or a pressure vacuum breaker; and other various kinds of backflow preventers in general.
The Safe Drinking Water Act was the turning point in that it put an awareness out there that caught the eye of manufacturers. This was really the key thing that happened. All of a sudden, in the 70’s the market expanded because of the Safe Drinking Water Act. People realized there was now volume associated with backflow preventers and, as such, manufacturers could offer competitive pricing and make a profit. They didn’t have to start with products available and make backflow preventers out of them. Originally backflow preventers looked more like water meters than backflow preventers because they were developed by water meter companies.
The Watts Regulator model 900 was key turning point in where we are today in backflow preventers. In our evolution of backflow preventers, this model (which is now an extinct product) served a key purpose. This product line brought the price of backflow preventers in a 1” size from $400 in the 70’s to below $200 today. It also brought the weights down from 40 pounds to under 10 pounds for the 1” size.
To better understand what happened in this evolution, I want to talk a little about product design and manufacturing technology. To do this I will focus individually on a few of the elements of the backflow preventers. There are things that all backflow preventers have in common: force loading for the check valves (this was always a requirement), seals and gaskets (again, always a requirement), many had and have diaphragms, all have bodies with pressure containing parts as they transport the water, resilient seated ( a key item that changed check valves into backflow preventers) and finally the wetted components inside that afford corrosion resistance. Each of these elements of the backflow preventer will be presented and how each one has evolved and caused this evolution.
The original force loading was lead and iron weights. This was the force loaded check valve. The force loading, when I entered the industry, was compression springs. At that time, the compression springs weren’t even contained. You needed a jack screw to take apart a 6” or 8” valve. Extension springs were one of the first evolutions that came into play, and then tension springs were employed.
The first seals and gaskets were leather which was considered to be resilient seated. Then asbestos was used for seals and gaskets. Other fibers have been used for gaskets since then. Today many of the seals and gaskets are elastomeric. Years ago were typically asbestos or fiber. Some of the big flange seals that were asbestos or gaskets actually are now hard elastomers. Thirty years ago elastomers that would do the job were not available. Probably the most innovated seal that has come along has been the“O” ring. “O” rings were slow to reach the plumbing industry because originally they were very expensive. They were designed for hydraulics and aerospace. Back in the 70’s you needed a good reason to use an “O” ring because an “O” ring cost maybe ten times what a rubber gasket cost. Due to the production methods and other factors, “O” rings now are nickels and dimes. “O” rings are definitely where the seals and gaskets have evolved to.
The first diaphragms I looked at were metal (stainless steel or copper) diaphragms. Next, diaphragms were from produced die cut materials such as rubber gaskets. This further evolved to using laminated sheets, a nylon material laminated with the elastomers. The laminated sheet gives the rubber its strength; its reinforcement. The next evolution in the diaphragm, beyond the simple sheet stock that was reinforced, was molded gaskets. Today, molded gaskets are what I would call state of the art diaphragms on backflow preventers. As stated earlier, as this industry has evolved, the backflow preventers have evolved because of the technology available to us. A little engineering trivia, if you calculate the surface area of a die cut diaphragm, it is only about 80% effective in the force it generates. The efficiency goes up in a rolling diaphragm to as high as 90%. What does this mean? It means you can reduce the size of the diaphragm by using a rolling diaphragm and still have the same power from it. In a backflow preventer this means we can reduce the size of the backflow preventer without changing function.
Next we will review body materials. Initial bronze was common on backflow preventers thru the 2” size valve and, in fact, when I started bronze was available up to 3”. Cast iron started at 4” and up. The first cast iron that I’m familiar with was protected with galvanizing. The first backflow preventer that I worked on was epoxy coated with silver so people would think it was galvanized because galvanized was considered “state of the art” at the time. As time went by, epoxy coating proved to be a superior coating. We don’t just use it on cast iron, one company even uses it on steel. Steel became one of the revolutionary things that was important in reducing the weight in large backflow preventers. However, the steel designs are now basically obsolete because of the evolution to stainless steel. The steel and the epoxy were important because manufacturers realized they could make bodies of backflow preventers from other than casting. This opened the way for more opportunities for designers fabricating stainless steel.
Stainless steel, which came next, was an evolution from plain fabricated steel. The advantage was the weight. At this point in time, the fabrication techniques were automated; robotics became a key factor and stainless steel became price effective with the others besides offering the lighter weight. It also offered the opportunity to do different designs because of fabrication in different shapes. It allows bigger and cleaner hollow sections than with castings.
The next material was plastics. An early “RP” was manufactured with a plastic body (not the whole body but the pressure containing portion). The bottom inlet and the bottom outlet shut off valves were bronze, but the actual body and relief valve were plastic.
Next is the evolution of seats. When I came into the industry, the majority of the seats whether it was a ¾” or 10” valve was bronze. Stainless steel became popular because bronze had erosion problems; especially as applications expanded beyond normal water works. Irrigation, for example, there is a much higher velocity and because they were not building to a plumbing code with reference to the velocity in the pipes. The backflow preventer is designed to work everywhere. There wasn’t a special design for one industry versus the other. Then the industry realized plastics could be molded into seats. Plastic seats could hold up to high velocity and to corrosion. Today plastic seats are “state of the art” all the way through 10” valves.
Way back the resilient part of the seats were die cut rubber; most of it being natural rubber. Slowly the industry realized they could do custom shapes. They didn’t have to be round and they could be shaped, to get a better clamping area for a better seal. The rubber material evolved from natural rubber to Buna and then chloramines forced the industry to evolve again. Today, for the seating material in most backflow preventers, you will find custom molded silicone rubber.
The evolution on the disc holders happened quickly. Some of the very early large ones were iron and epoxy coated. You would be hard pressed to find them today. As people realized they needed to be maintained and work on backflow preventers they needed something better. Bronze was the natural choice. Bronze, at that time was 85% copper. Today it is about 81% copper. Bronze is not a good alternative today, price wise. Stainless steel became an option for some people. I would say that plastic is now the “state of the art”. Thirty years ago it would have hard to find a plastic that would hold up to the pressures or shock waves. Through the evolution in the plastic industry, we are now able to adopt a lot of the plastics and bring it into the backflow preventer.
Bronze to stainless steel was a common material for sliding parts. Different materials were tried to prevent galling when they rubbed against each other. When it was metal, it was bronze and stainless steel. Today you will see mostly plastic. Plastic has the ability to not only slide well with metal components, but also with other plastic parts. You can actually combine different plastics with different properties to rub against each other. You can find glass filled material as a bushing for a non-glass filled material.
A key thing with plastic is to use the right plastic with the right application. All plastics do not work well in the water. All plastics don’t like chlorine. All plastics don’t like to be in tension and some plastics like to be in compression. Polysulphone is unique as it is a very high temperature plastic. Quite often you will find polysulphone components in backflow preventers rated above, or close to 180 ºF. There are few problems when it is exposed to certain chemicals. Plastic will always be there but you will never see the plastic part of a backflow preventer come down to one material as fitting all needs. There will always be a “laundry list” for specific applications.
As the Safe Water Drinking Act came in and the awareness for backflow protection increased, the volume for backflow preventers increased. This allowed manufacturers to design different backflow preventers for specific applications. As was mentioned, the original ones were designed for all applications. Specific backflow preventers for plumbing came next with special backflow preventers for irrigation close behind. The newest addition for special designs for backflow preventers is fire protection. Fire protection really spurred the manufacturers to do something because the fire protection industry is accustomed to having things that are different from the plumbing industry. For example, fire protection is accustomed to having different connections. Rarely were there butterfly valves on backflow preventers until the fire protection industry said they are great because they are smaller, easier to carry to the sites and they are equipped with indicating stops such as OS&Y valves. The difference in weight between and OS&Y valves on the inlet and the outlet on a 10” backflow preventer is almost a thousand pounds per valve. Part of the evolution on the water side was brought about by the fire protection industry. The water industry is taking advantages of some of the things being done by the fire protection industry such as elbows being integrated in the backflow preventer. Years ago you would assume the plumber installed the elbows. Today the backflow preventer is tested and certified with the elbows as part of the assembly. The test for a backflow preventer is from gate valve to gate valve. We have reduced the pressure loss in the plumbing systems because the pressure loss in the elbows has to be assimilated by the overall head loss in the backflow preventer.
There is even a market for temporary backflow preventers. There are backflow preventers produced for construction sites that have temporary water hook ups and what brought in the ½” backflow preventers for “point of use”. As the volume for backflow preventers increased it resulted in dramatic size reductions and dramatic weight reductions. Another way the manufacturers have been able to do size and weight reduction is to include shut off valves as part of the assembly. Butterfly valves are actually built into the assembly. By having the shut off valves as part of the body, the backflow preventer is about as compact as you can get. Manufacturers are also able to inventory a single backflow preventer not knowing where the flow is going to come from, such as from the ground or the right or the left. This makes it easier for the plumber to install. Multiple orientations is a tremendous innovation. Some brands provided an adjustable link right in the middle of the backflow preventer between the first and second check where you can loosen, swing and point it in a different direction. All original large backflow preventers had two covers for each check. In some designs, going to a single cover has dramatically reduced the size of the valve.
Today, in my opinion, if a manufacturer states they are a “state of the art” manufacturer, they have available, as separate sizes, the ½”, ¾”, 1”, 1½” and 2”. Each one of these sizes are separate valves, separate internal components, sized for efficiency and readily available from most manufacturers. They have “O” Seals, plastic components and multiple orientations.
A summary on today’s state of the art for sizes 2½” though 12”. The minimum body sizes are 4”, 6”, 8” and 10”. These are separate, individual sizes. I don’t list 2½” and 3 because the 4” valves “state of the art” design, are so small and so light weight there is no advantage to carry a 3 inch valve with separate parts. The 4 inch valve is truly a small valve. There are 10 inch valves that have low enough head loss that they actually meet the 12 inch requirements. If there is a need for a 12 inch (I do believe there is an actual 12 inch valve on the market) you can get a 10 inch valve that will meet that need. I believe stainless steel is the “state of the art” for bodies. I designed with epoxy coated parts. It is good, but not for a lifetime. Its hard to get o-rings to seal once the epoxy has been damaged. Wetted components all the way up through 10 inch are plastic to plastic. Engineers are able to design with plastic so there is reason to go back to the metal components. Plastic is lighter, especially on the larger components and are more cost effective than bronze. O rings come in all sizes from a 32nd of inch all the way to ¼ inch. There is no reason to use bolted down gaskets where you have to worry about bolt torque or uniform tightening Pressure reactive “o” rings seals where the water pressure is doing all the work for you is the way to go today. Poppet designs will never be as small or as easy to take apart as a swing or cam check. Size and safety are two reasons to use torsion springs. Torsion springs, I feel, are safer in that they physically only move ¼ turn. It doesn’t matter how big the torsion spring is it can only come ½ way undone. A compression spring that is 6 inches can expand to 12 inches which just the right height to hit a person in the jaw. As an engineer and you calculate things out, you can get more power per pound in a torsion spring than you can in a compression spring. Thanks to our friends in the fire protection industry, grooved flanges are much more acceptable in the water work side. We are using the grooved flanges for covers on backflow preventers with joints between 1st and 2nd checks. They are smaller and lighter weight and much easier to seal with a soft rubber gasket rather than a bolted flange. Multiple orientations should be available.
The need for safe water, awareness of the relationship between cross-connections and illness and changing manufacturing technology will continue to create evolution of backflow prevention design.
Article by Rand Ackroyd first appeared in ASSE’s Working Pressure magazine