The following recommended sizing procedure for grease interceptors may be used by the engineer as a general guideline for the selection of these units. The designer should consult the local ad- ministrative authority regarding variations in the allowable drain-down times acceptable under the approved codes. Calculation details and expla- nations of the decisionmaking processes have been included in full in the examples as an aid to the designer using these guidelines in spe- cific situations.
Example 8-1 Single-fixture installation, manu- factured unit, no flow control. Size the grease interceptor for a three-compartment pot (scul- lery) sink, each compartment 18 in. × 24 in. × 12 in. (457.2 mm · 609.6 mm · 304.8 mm).
Step 1 Determine the sink volume.
Cubic contents of one sink compartment = 18 × 24 × 12 = 5184 in.3
Cubic contents of the 3 sink compartments = 3 × 5184 = 15,552 in.3
Contents expressed in gal = 15.552 = 67.3 gal
(457.2 · 609.6 · 304.8 = 84.95 · 106 mm3 = 8.49 · 104
mL.
3 · 8.49 · 104 = 25.44 · 104 mL
Contents expressed in liters = 254.4 L)
Step 2 Determine the required size of grease
interceptor. A sink (or fixture) is seldom filled to the brim, and dishes, pots, or pans displace ap- proximately 25% of the water. Therefore, 75% of the actual fixture capacity should be used to es- tablish the drainage load.
0.75 × 67.3 gal = 50.8 gal (0.75 · 254.4 =190.8 L)
Step 3 Calculate the flow rate based on deten-
tion time. The most generally accepted drainage period is 1 minute. On any given project, how- ever, conditions may exist whereby a longer drainage period could be considered. The flow rates are calculated using the following equa- tion:
Drainage load, in gal (L) = Flow rate, in gpm (L/s) Drainage load, in min (s)
Therefore, the flow rate for this example would be as follows: 50 gpm (3.15 L/s) for a 1- min total drainage or 25 gpm (1.58 L/s) for a 2-min total drainage (this condition provides a longer period, should one be desired).
Step 4 Select the grease interceptor.
A grease interceptor with a rated capacity of 50 gpm (3.15 L/s) for 1 min or 25 gpm (1.58 L/ s) for 2 min should be selected. The local ad- ministrative authority having jurisdiction should be consulted. Choose an on-slab system, if pos- sible, for ease of installation and accessibility if a semiautomatic device is used.
Example 8-2 Manufactured unit, multiple-fix- ture installation with flow control. Size the grease interceptor for the fixture sizes and the flow rates shown below:
1. A dishwasher with a 37-gal (140-L) power wash tank and a 7.5-gpm (0.5-L/s) final rinse consumption. The power wash tank is dumped after each meal serving, 3 times a day. The maximum final rinse consumption is 450 gph (0.5 L/s).
2. A grease-extracting hood 20 ft (6.1 m) long. Hood is a single section with a single drain washed automatically once per day on a 5-
minute cycle.
3. The floor drains: 2 floor drains, each one 4 in. (101.6 mm) in size and rated at 50 gpm (3.15 L/s) flow capacity.
Step 1 Calculate the flow produced from each
fixture.
1. The dishwasher The engineer should refer to the manufacturer’s literature. Peak drain flow (initial rate) with full tank = 38 gpm (2.4 L/s). Use 40 gpm (2.5 L/s) as a conservative figure.
2. The grease-extracting hood The engineer should refer to the manufacturer’s literature. The total water consumption is equal to the total water of the nozzles in the duct plus the total water consumption of the drain line preflush. Number of nozzles required in a 20-ft (6.1-m) long duct = 2L + 4 (from the manufacturer’s literature), where L is equal to the length of the hood. Therefore, the to- tal number of nozzles is equal to 2 (20) + 4 = 44.
From the manufacturer’s literature, the flow rate per nozzle (at 50 psi [344.7 kPa] flowing pressure) is equal to 0.28 gpm (0.02 L/s) per nozzle.
Therefore, the total flow is equal to the num- ber of nozzles multiplied by the flow rate per nozzle = 44 × 0.28 = 12.32 gpm (44 · 0.02 = 0.88 L/s).
Add flow from the preflushing line.
From the manufacturer’s literature, the preflushing flow rate at 50 psi (344.7 kPa) flowing pressure = 1 gpm (0.06 L/s).
Therefore, the total water consumption and the maximum drain flow = 12.32 + 1.00 = 13.32 gpm (0.84 L/s).
As a conservative figure, the engineer should use 1 gpm per lineal foot (0.21 L/s per meter) of hood = 20 × 1 = 20 gpm (1.3 L/s).
3. The floor drains as listed, flow rate = 50 gpm (3.15 L/s) for each drain.
Step 2 Establish the expected maximum flow
rate with diversity.
It is highly unlikely that the dishwasher, the grease-extracting hood, and the two floor drains will ever discharge simultaneously. It is possible,
however, that the dishwasher and the grease- extracting hood may discharge simultaneously at the end of a day’s operation. If this is the case, the peak flow would be 40 gpm (2.5 L/s) for the dishwasher plus 20 gpm (1.3 L/s) for the hood = 60 gpm (3.8 L/s).
Step 3 Decide whether separate units for each
fixture or a single unit for multiple fixtures will be used.
The proximity of the grease interceptor to the grease-discharging fixture is very important for cost savings in venting and for ensuring that the discharge liquid temperature stays above 120°F (48.9°C) to maintain the grease in a liquid state into the interceptor. A single unit for multiple fixtures means a recessed grease interceptor be- cause of the slope required between the fixture and the grease interceptor. On the other hand, a single unit is generally more economical as an overall installation. Use a single recessed unit to accommodate all fixtures whenever possible.
Step 4 Decide where the flow control devices
are to be installed.
The dishwasher produces the largest waste flow from a single fixture. If a flow control device is included to limit the dishwasher discharge to 20 gpm (1.3 L/s), then the total flow will be 20 + 20 = 40 gpm (2.5 L/s). A flow control device can- not be used on the extractor hood because the hood might flood during the discharge if the flow control device limits the drain flow to less than the nozzles produce.
The flow control devices are not generally practical to use on the floor drains because of the difficulty of the location in the floor. It is best to install the dishwasher flow control de- vice at the dishwasher above the floor. If the flow control device were located at the grease inter- ceptor, there would be a chance of flooding the floor drain line because of the common connec- tion to the grease interceptor.
An alternate solution would be to connect the floor drains separately to the grease inter- ceptor and use a flow control device at the grease interceptor for the combined dishwasher and ex- tractor hood line. A common discharge line will be used in this example.
Step 5 Size the grease interceptor.
The largest unrestricted flow rate will still be 50 gpm (3.2 L/s) from the floor drains rather than the 40 gpm (2.5 L/s) from the simultaneous
discharge of 40 gpm (2.5 L/s) from the dish- washer and the extractor hood combined together. A 50 gpm (3.2 L/s) unit should be se- lected.
Step 6 Decide if the unit is to be manual or
semiautomatic.
Step 7 Miscellaneous considerations.
In this example, it is very important that the cover does not become too hot to walk over and that the cast iron unit does not rust and cor- rode in the slab on the grade installation. A concrete recess should be provided to help avoid these problems.
Example 8-3 Outside custom-made unit, mul- tiple-fixture installation, no flow control. Size the grease interceptor for the fixture sizes and the flow rates as in Example 8-2 and the particles of 0.01 in. (0.25 mm). Assume ρ1 = 0.909.
From Example 8-2, the following data are obtained:
dishwasher flow = 40 gpm (2.5 L/s) grease-extracting hood = 20 gpm (1.3 L/s) floor drains = 50 gpm (3.2 L/s)
Step 1 Establish the expected maximum flow
rate with diversity.
The diversity consideration will be the same as that shown in Example 8-2 except that no flow control is to be used in this example to en- sure that all the grease-laden waste gets to the outside grease interceptor before solidifying. In this case, the peak flow rate with the fixtures discharging simultaneously will be 60 gpm (3.8 L/s).
Step 2 Calculate the required volume for the
minimum detention period, P.
Assume the minimum detention period, P, to ensure the solidification of grease to be 5 min- utes, which is also equal to the flow-through period. Then, the minimum volume is equal to 5 × 60 = 300 gal (1135.5 L).
Step 3 Select and check the tank size.
Assume the practical size for cleaning out the grease and accessibility to be 5 ft long, 2 ft wide and 5 ft liquid depth (1.5 m · 0.6 m · 1.5 m). The volume is equal to 50 × 7.48 = 374 gal (1415.6 L).
Step 4 Calculate the horizontal velocity.
V = = = 0.8 fpm (4.1 m/s)
Step 5 Check the overflow rate.
Overflow rate, v = = 0.8 × 5/5 = 0.8 fpm (4.1 m/s)
This overflow rate provides 100% particle re- moval down to 0.1 in. (3.0 mm), which is not sufficient. However, we can improve the removal efficiency by lowering the overflow rate. This con- dition is accomplished by enlarging the plan area and reducing the depth because the depth has almost no dependence on the tank efficiency. Therefore, it is assumed a 5 ft long × 5 ft wide × 2 ft liquid depth (1.5 m · 1.5 m · 0.6 m).
The detention time and horizontal velocity remain the same but the overflow rate is now
0.8 × 2/5 = 0.32 fpm (1.6 m/s)
With an overflow rate of 0.32 fpm (1.6 m/s), a 100% removal efficiency of particles down to 0.01 in. (0.2 mm) is acceptable.