By C.F. ‘Chubb’ Michaud, CWSI-VI
In Part 4 of this series, we look at how pressure drop is measured when water flows through valves. How do valves differ? We will also look at the meaning and purpose of using fixture-count determinations for sizing pipes and equipment.
Pressure drop in valves
Control valves are not the same as pipe but they do add measurable ∆P to a system. The pressure loss comes from the irrecoverable loss of energy due to friction as water flows. Valve manufacturers measure the pressure losses at various flows and give the valve a rating called Cv (ceevee). The Cv of any valve is the flow, in gpm, to a one-psi ∆P. The same relationship between ∆P and flow holds true. How do you calculate the ∆P through a valve with a Cv = five (a typical residential control valve) that you intend to flow at 15 gpm? Divide the intended flow by the Cv (15/5 = 3) and square the result (32 = 9). The ∆P through the valve alone at the target flowrate (15 gpm) flow is nine psi. Do you have the right valve for this job?
A serious ∆P problem could be encountered if you built a valve nest with full-flow ball valves that you might convert to an automated system down the road, using diaphragm valves. For example, the Cv of the ball valve is around 90; running the system at 45 gpm means you would only see a ∆P of 0.25 psi per valve (in series) (1/2 the flow = 1/4 the ∆P). Later, you convert to an air-operated diaphragm system (Cv = 15) and develop (45/15)2 = 9 psi per valve. If you are trying to backwash a 30-inch multimedia filter at 90 gpm, you could have done it with the ball valves and only expect 2 psi from the valves (there would be two in series). When you go to the diaphragms, however, you find (90/15)2 = 36 psi x 2 = 72 psi in ∆P. Even though you could squeak by with a one-inch system using ball valves, you have to move up to a two-inch system with the automation. It pays to think ahead.
Similarly, when you plan the system size, you must look ahead to the possibility that additional equipment may later be added. You may have to protect a softener with an iron filter upstream and, if chlorine is used to oxidize the iron, a carbon filter may also be required. This will drain available operating pressure unless the filters are properly sized from the start.
In testing for NSF certification, the maximum flow for a filter is measured as the flow attained at 15 psi ∆P across the filter. This total pressure drop is a combination of ∆P in the pipes, filter bed, internals and the valve. It does not include the run length of the feed pipe or elevation change. Multiple filters will have a cumulative ∆P equal to the sum of all of the individual ∆Ps. Older valves with small risers can shave precious psi figures off your filter and cause a rating loss of a couple of gpm in the certified flows.
Trying to push 10 gpm through a filter that is rated at 7.5 will result in a pressure loss of almost 27 psi. With peak flows of 15 gpm, the pressure demand will approach 60 psi… just through the filter!
The Fleck 2510, represented as a 0.75-inch valve, has a Cv of 4.8. It shows 1 psi ∆P at 4.8 gpm; we can calculate that the maximum flow (at 15 psi) would be (15)1/2 x 4.8 or 4.8 x 3.9 = 19 gpm and a peak flow (at 25 psi) of 4.8 x (25)1/2 = 4.8 x 5 = 24 gpm. The Fleck 7000, a 1.25-inch valve, has a Cv of 9.2; the 2850 (1.5 inch) is 13.2. The 2900 (two-inch) is 27.5 and the three-inch 3900 has a Cv of 65. These numbers do not take into account the ∆P through the piping, the media bed and distribution system, so the actual performance will be lower than the valve rating.
Full-flow ball valves do not appreciably increase ∆P when fully open. Valves designed to restrict flow, however (such as gate valves needle valves, hose bibs and the like), will drop line pressure by as much as 15 feet of head pressure (about six psi). If you are using a flow-restricting valve to limit flow to a system, put it on the downside of the system rather than the feed side, so it does not interfere with inlet pressure available for backwashing or chemical draw through eductors.
Fixture count sizing
No discussion on flow would be complete without at least some mention of fixture count. The purpose of using fixture count is to properly size the service-line piping for a residence or place of business. Fixture count was developed by the American Water Works Association (AWWA) in 1975 and is presented in AWWA’s M-22 Manual. The idea is that you count the total number of fixture types, assign a value to each category and determine the total number of fixture units. It is assumed that piping will be sized to handle all fixtures simultaneously—a highly unlikely event. Then, using a chart, you can determine the flowrate required and thus, the correct pipe size.
I did this for my own residence and a fixture count for it can be seen in Table 4. I came up with a value of 37 fixture units and a flowrate of 25 gpm. From Part 2, Table 1, this would dictate a pipe size of 1.25 inches because a one-inch would be undersized. I live in an older home built in 1965. I have a 5/8ths meter and 0.75-inch pipe; everything seems to work just fine. A 0.75-inch line is only rated at about nine gpm. If the house was built today, it would have 1.25-inch plumbing with a four-cubic-foot softener and my double-headed shower on the second floor would deliver a brisker flow. Why such a difference?
The answer is that residential plumbing is not designed for maximum potential flow but for maximum probable flow. It is proportional to the number of fixtures that are likely to be used simultaneously. This ratio differs in commercial buildings and hotels where it is possible to approach a theoretical peak flow for short periods of time, such as early morning or lunch time. A study contracted by WQA in 1999 (www.aquacraft. com/sites/default/files/pub/Aquacraft-%282001%29- Analysis-of-Residential-Indoor-Peak-Demands-1.pdf) placed recording meters on several hundred homes to track flows and times. The typical flowrate recorded for any one home was 2.5 gpm with only an occasional excursion to the five-gpm mark.
Different residences with similar fixture counts may have widely different flow demands. Flow is a very site-specific need. Inlet plumbing size will be based on local building codes and will be very similar from place to place. More than likely, it will follow the fixture-count projections and seem greatly oversized. A home with five bathrooms will dictate larger plumbing, regardless of the number of occupants, as occupancy can change with time. Unfortunately, filtration equipment will be sized according to the same overly generous flow estimates. This can result in oversized equipment that doesn’t function well at the low flows actually encountered, further resulting in filter flow channeling and backwash flowrates that would overwhelm the drain line. Splitting the flow with parallel systems will address the flow concept design flaw. More on this later.
Estimate the supply demand by totaling the fixture units from the Water Customer Demand Data Table—Fixture Unit Count (Figure 3) and then by reading the corresponding ordinate. Read the chart vertically at the fixture count value, then horizontally and read your water flow demand in gpm.
Fluid flowing through pipes, fittings and valves uses energy in the form of pressure. In sizing systems, it is very important to take specific water demands into account. Static pressure in a water supply is not the same as dynamic pressure at the point of use. A 60-psi city water supply that has to travel through 100 feet of 0.75-inch copper pipe to get to a filter designed to run at 10 gpm will only deliver about 47 psi to the inlet of the filter.7 One has to keep available dynamic pressure in the forefront when designing filter systems using multiple in-line tanks. Stringing four units together in series can result in pressure losses of 30 psi or more at the point of use.
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