Piping Calculations
As a fluid flows through a piping system, it will experience a headless depending on, among other factors, fluid velocity, pipe wall smoothness and internal pipe surface area. The Tables on pages g and 10 give Friction Loss and Velocity data for Schedule 40 and Schedule 80 thermoplastic pipe based on the Williams and Hazen formula.
H=0.2083 X (100/C)l.852 X (ql.852/d4.8655)
Where: H = Friction Head Loss in Feet of Water/100 Feet of Pipe
C = Surface Roughness Constant (150 for all thermoplastic pipe)

q = Fluid Flow (gallons/min.)
d = Inside Diameter of Pipe

Fittings and valves, due to their more complex configurations, contribute significant friction losses in a piping system. A common method of expressing the losses experienced in fittings is to relate them to pipe in terms of equivalent pipe length. This is the length of pipe required to give the same friction loss as a fitting of the same size. Tables are available for the tabulation of the equivalent pipe length in feet for the various sizes of a number of common fittings. By using this Table and the Friction Loss Tables, the total friction loss in a plastic piping system can be calculated for any fluid velocity.

For example, suppose we wanted to determine the pressure loss across a 2″ Schedule 40, goo elbow, at 75 gpm. From the lower table we find the equivalent length of a 2″ goo elbow to be 5.5 feet of pipe. From the Schedule 40 Pipe Table we find the friction loss to be 3.87 psi per 100 feet of pipe when the flow rate is 75 gpm. Therefore, the solution is as follows:
5.5 Feet/goo Elbow x 3.87 psi/100 Feet
= 0.21 psi Pressure Drop/goo Elbow

which is the pressure drop across a 2″ Schedule 40 elbow. But, what if it were a 2″ Schedule 80 elbow, and we wanted to know the friction head loss? The solution is similar, except we look for the friction head in the Schedule 80 Pipe Table and find it to be 12.43 feet per 100 feet of pipe when the flow rate is 75 gpm. The solution follows:
5.5 Feet;go0 Elbow x 12.43 Feet/100 Feet
= 0.68 Feet Friction Head/goo Elbow

which is the friction head loss across a 2″ Schedule 80 elbow.
For a copy of the tables mentioned in this section, please contact customer service.

Valve Calculations
As an aid to system design, liquid s1Z1ng constants (Cv values) are shown for valves where applicable. These values are defined as the flow rate through the valve required to produce a pressure drop of 1 psi. To determine the pressure drop for a given condition the following formula may be used:
P= (Q2S.G. )/(Cv2)
Where: P = Pressure drop across the valve in psi

Q = Flow through the valve in gpm
S.G. = Specific gravity of the liquid (Water=l.0)
Cv = Flow coefficient

See the solution of the following example problem. For Cv values for specific valves, contact customer service or consult the manufacturers catalog.

EXAMPLE:
Find the pressure drop across a 1 1/2″ PVC ball check valve with a water flow rate of 50 gpm. The Cv is 56.

P=(502 X 1.0)/562
P=(50/56)2
P=Q,7g7 psi

Hydraulic Shock
Hydraulic shock is the term used to describe the momentary pressure rise in a piping system which results when the liquid is started or stopped quickly. This pressure rise is caused by the momentum of the fluid; therefore, the pressure rise increases with the velocity of the liquid, the length of the system from the fluid source, or with an increase in the speed with which it is started or stopped. Examples of situations where hydraulic shock can occur are valves which are opened or closed quickly or pumps which start with an empty discharge line. Hydraulic shock can even occur if a highspeed wall of liquid (as from a starting pump) hits a sudden change of direction in the piping, such as an elbow.

The pressure rise created by the hydraulic shock effect is added to whatever fluid pressure exists in the piping system and, although only momentary, this shock load can be enough to burst pipe and break fittings or valves.

Proper design when laying out a piping system will limit the possibility of hydraulic shock damage.
The following suggestions will help in avoiding problems:

1. In a plastic piping system, a fluid velocity not exceeding 5 ft./sec. will minimize hydraulic shock effects, even with quickly closing valves, such as solenoid valves. (Flow is normally expressed in GALLONS PER MINUTE-GPM. To determine the fluid velocity in any segment of piping the following formula may be used:
V=(0.4085xGPM)/Di2
Where: v = fluid velocity in feet per second
Di = inside diameter
GPM = rate of flow in gallons per minute

Flow Capacity Tables are available for the fluid velocities resulting from specific flow rates in Schedule 40 and Schedule 80 pipes. The upper threshold rate of flow for any pipe may be determined by substituting 5 ft./sec. Fluid velocity in the above formula and solving for GPM. Upper Threshold Rate of Flow (GPM) = 12.24 Di2

2. Using actuated valves, which have a specific closing time, will eliminate the possibility of someone inadvertently slamming a valve open or closed too quickly. With air-to-air and air-to-spring actuators, it will probably be necessary to place a flow control valve in the air line to slow down the valve operation cycle, particularly on valve sizes greater than 1 1/2″.

3. If possible, when starting a pump, partially close the valve in the discharge line to minimize the volume of liquid that is rapidly accelerating through the system. Once the pump is up to speed and the line completely full, the valve may be opened.

4. A check valve installed near a pump in the discharge line will keep the line full and help prevent excessive hydraulic shock during pump start-up. Before initial start-up the discharge line should be vented of all air. Air trapped in the piping will substantially reduce the capability of plastic pipe withstanding shock loading.

Shock Surge Wave
Providing all air is removed from an affected system, a formula based on theory may closely predict hydraulic shock effect.
Where: p = maximum surge pressure, psi
then p = vC

EXAMPLE:
A 2″ PVC Schedule 80 pipe carries a fluid with a specific gravity of 1.2 at a rate of 30 gpm and at a line pressure of 160 psi. What would the surge pressure be if a valve were suddenly closed?
From table: c = 24.2 v = 3.35 

p = (3.35) (26.6) = 90 psi

Schedule 80 2″ PVC has a pressure rating of 400 psi at room temperature. Therefore, 2″ Schedule 80 PVC pipe is acceptable for this application.

CAUTION: The removal of all air from the system in order for the surge wave analysis method to be valid was pointed out at the beginning of this segment. However, this can be easier said than done. Over reliance on this method of analysis is not encouraged. Our experience suggests that the best approach to assure a successful installation is for the design to focus on strategic placements of air vents and the maintenance of fluid velocity near or below the threshold limit of 5 ft,/sec.