Answering questions on water hammer, valve sizing and more.

I receive a number of interesting questions regarding design, operation and performance in a plumbing system. Some of these questions are worth sharing with everyone else. These questions all relate to valves. So here they are:

"I hear water hammer when I turn on the faucet. I thought water hammer only occurred when you turn off the faucet?"

Well, you heard a noise, but it wasn't water hammer. We tend to call it reverse water hammer. I know, that is a strange term and you may be asking what I mean.

Water hammer does occur when there is a quick closing of a valve. The shock wave will actually occur any time there is an instantaneous change in velocity. The water doesn't have to completely shut off for hydraulic shock to occur.

When you instantaneously turn on the water, you suddenly increase the velocity of flow. In that operation, you can create a vapor pressure in the valve area. That is a fancy engineering term to say that the water wants to flow real fast and little air pockets (vapor) can form.

(For you engineers out there, I realize that this is an oversimplification of a complex occurrence.) The noise often sounds like a clunk, sometimes it is chattering. But it is different than the sound of water hammer.

Since both water hammer (hydraulic shock) and vapor pressure are related to the velocity, you can see why it is called reverse water hammer.

The next question is how to get rid of this noise from vapor pressure. Some contractors have found that water hammer arrestors also work for vapor pressure occurrences. They do, in fact, work because they are, again, balancing the energy in the water piping system. The other means of correcting the problem is the proper sizing of the valves and balancing of the water flow in the system.

"A manufacturer suggested we use a ball valve to regulate flow. Is that correct?"

No. Technically, the manufacturer is wrong. A ball valve is intended to be a full-open, full-close type of valve. Similar to a gate valve, a ball valve is not designed to control the flow of water in a piping system. In fact, you can damage the ball valve with prolonged use of the valve as a modulating valve.

I read the manufacturer's literature with the description of an in-line ball valve for controlling flow. I understood why they suggested a ball valve, but I disagree with their recommendation.

The manufacturer shows the ball valve on a main line of the water distribution system connecting to a control valve. Its concern is the potential conflict with the plumbing codes. Most plumbing codes have a requirement for full-open valves in the main piping. But, the purpose of those valves is to shut the water on or off. These full-open valves are not for regulating the flow.

So the manufacturer is wrong. A globe valve or butterfly valve should have been recommended. When the flow in the piping system must be modulated, you do not use incorrect valves. The installation of these in-line, non-full-open valves would not violate the plumbing code.

Remember the code is discussing the required valves for isolating the flow, not valves for regulating the flow to still be within the design requirements for the water distribution system. I will admit that this is an area where the plumbing codes could use some work so as not to result in the wrong use of a valve.

"I need a 4-inch backflow preventer for the connection to a piece of equipment. With the cost being so high, could I put two 2-inch backflow preventers in a manifold to replace the one 4-inch valve?"

Absolutely not! OK, everyone knows that 2-inch plus 2-inch doesn't equal 4-inch. First, let's look at the flow characteristics. In a 4-inch line, you can flow about 320 gpm. In a 2-inch line, you can flow about 80 gpm. Adding 80 gpm plus 80 gpm does not equal 320. In fact, it would take about four 2-inch valves to equal the flow through a 4-inch valve.

The other problem associated with manifolding any valve is the balancing of the valves. All valves are not created equal. If you install two identical valves in a manifold, one is going to be doing the bulk of the work. In other words, one control valve will be slightly more sensitive to the pressure differentials. Hence, it will be the first to open. That valve will allow the flow until the other valve reaches the pressure setting to open.

I have found that, for control valves, it is better to analyze the flow parameters. If there is a period of low flow and a period of higher flow, I size manifold control valves for that application. In other words, rather than putting in two identical valves, I put in one valve of a size for the low-flow condition, and the other valve for the combined higher flow.

The result may be a 1-inch control valve with a 2 1/2-inch control valve. By control valve I mean any valve used to regulate flow, such as a pressure-reducing valve or a backflow preventer.

Don't ever expect two identical control valves to work the same together. While it looks good on paper, it just won't happen in reality.

"Why don't flush valve manufacturers set the valve correctly in the factory?"

I laughed when I received this question. The first thing that entered my mind was, "Now the manufacturers have to be mind readers."

Of course, the setting of the flush valve is completely dependent on the pressure of the water distribution system. Since that pressure can range from 20 psi to 80 psi, there is no way for the manufacturers to adjust the valve setting in the factory.

By the way, that question did not come from a plumbing professional. But I had to add it so you would get a laugh, too.

Julius Ballanco At ISH NA 2003

Julius Ballanco is a scheduled speaker at this year's ISH North America Tradeshow held Oct. 1-3 in Las Vegas, Nev. He will present two topics on Wednesday, Oct. 1: "Major Code Revision on Drainage and Venting" at 1:30 p.m.; and "Water and Backflow Plumbing Code Update" at 3:30 p.m. To register for the show, visit www.ish-na.com.