SUBSTATION CIVIL ENGINEERING GUIDE CEG-3

Subject:

/

Spill Prevention Control and Countermeasure (SPCC)

Oil Retention Basin Spreadsheet User’s Guide

Approved By: / SED Civil / DFW / Revision: / 1
Effective Date: / 02/10/05 Electronically Approved /

Page:

/

1 of 14

1.0 PURPOSE

This design guide explains how to use the SPCC Oil Retention Basin Spreadsheet to analyze existing SPCC containment systems and to design new containments. This supplements the SPCC Plans and Facility Improvements Design Criteria Memorandum (DCM No. C-1.3 Rev. 1). SPCC facility improvements decrease the probability of an oil spill discharging into navigable waters.

2.0 SCOPE

The SPCC Oil Retention Basin Spreadsheet (Spreadsheet) contains two worksheets, “Existing/New SPCC Basin Storage” and “Existing/New SPCC Flow-Through Analysis”. “Existing/New SPCC Basin Storage” computes the spill retention capacity in an existing basin and checks the storage adequacy for a new basin design. “Existing/New SPCC Flow-Through Analysis” checks the storm water flow-through requirements. The application of this user’s guide for specific projects shall be as directed by the Responsible Engineer (RE).

3.0 GLOSSARY OF TERMS

Above-base storage
/ Volume of oil stored above the highest floor elevation.
Allowable Oil Storage
/ Volume of oil that can be contained in the basin.
Back Wall
/ Back wall in weir system.
Base Storage
/ Volume stored below the highest floor elevation.
Basin Capacity
/ Volume of fluid that can be contained in the basin.
Basin Floor
/ Rectangular floor at bottom of basin.
Bermed Area
/ Area enclosed within the berm system.
Cut-off Wall / (see SkimmerWall)
Highest Point
/ Highest basin floor elevation compared to top of weir elevation.
Lowest Point
/ Lowest basin floor elevation compared to top of weir elevation, typically at the weir location.
Oil Specific Gravity / The specific gravity of the equipment mineral oil is set at 0.85, the nominal value for transformer oils.
Overflow Weir / Wall that regulates the outflow rate from and the maximum water level within the oil retention basin.
Perimeter Wall
/ Outermost wall of weir system.
Skimmer Wall / Wall that regulates the flow and separation of water and/or oil upstream of the outlet structure.
Sluice Slot
/ Flow passage below the skimmer wall.
Spill Quantity
/ The volume of oil and/or water to be retained. It is equal to the maximum of the following (based on reference 3, Section 5.2.3):
1)110% of oil volume of largest container
2)10% of total aggregate oil volume (<100,000 gal)
Weir / (see Overflow Weir)

4.0 ANALYSIS CASES

Note: /
  • Existing basins: Analyze Cases A and B below to determine the allowable oil storage of the existing SPCC system based on site-specific conditions.
  • New basins: Analyze all three cases.
  • Refer to Sections 5.1.3.2 and 6.1.2 for definitions of variables in the figures below.
  • Basins in areas of high water table should be checked for buoyancy.

4.1 CASE A: Oil spill into empty basin with no overflow

  • Analyze/design the basin or berm system to contain the maximum of the following:

1) Spill quantity equal to 110% of oil volume in largest container

2) Spill quantity equal to 10% of total aggregate oil volume (<100,000 gal)

4.2 CASE B: Oil spill when existing basin water level is at top of overflow weir

  • With the basin full of water, analyze/design the basin or berm system to contain the spill quantity listed in Section 4.1.
  • Because the oil has a specific gravity of 0.85, the spilled oil remains behind the skimmer wall and above the existing water level, which is to the top of overflow weir.
  • This condition determines the upper limit of the underflow weir. To prevent the oil from mixing with water, a pressure balance between the fluids must be maintained. The pressure balance is defined by Z′= (d12-A- C ′)/0.85 + C′, where C′ is the user defined freeboard (refer to section 6.1.2 for definition of d12).
  • The following two conditions are used to maintain the pressure balance:

1) Condition 1: If Z′< B – C′

2) Condition 2: If Z′B – C′, a required freeboard, Creq, must be provided to prevent oil from flowing over the skimmer wall (refer to section 6.1.3 for definition of Creq).

4.3 CASE C: Storm overflow only,no oil spill

  • Design all basin or berm systems to pass the design storm (25-year return period) runoff in combination with no oil.
  • The design should include a basin or berm dewatering system. The dewatering system may be gravity or pump operated. The system (pipes, valves, pumps, or other devices) should be capable of dewatering a full basin or berm within one hour or less.
  • The outlet flow capacity should exceed the design storm runoff (see Section 5.2.5 for outlet variables).

5.0 USER INPUT

Note: A star appears to the right of an input value when the input does not meet the normal minimum requirement (see Section 6.1.3). A larger value must be entered.

A pound sign appears in the input cell or to the right of an input cell when an invalid number (either too large or too small) is entered. Check to make sure that the correct input values were entered.
5.1 Sheet 1: Oil Storage Input

Note: Sheet 1, “Existing/New SPCC Basin Storage”, calculates the amount of oil to be contained and the allowable oil storage.

5.1.1 Select Spreadsheet Type

Note: Check one of the following options. If both or none of the options have been selected, the spreadsheet title will print “Check Spreadsheet Selection”.

Existing Basin Analysis= Select this option if you are analyzing an existing basin. The Sheet 1 title will print “Existing SPCC Basin Storage” and Sheet 2 will print “Existing SPCC Basin Flow-Through Analysis”.

New Basin Design= Select this option if you are designing a new basin. The Sheet 1 title will print “New SPCC Basin Storage” and Sheet 2 will print “New SPCC Basin Flow-Through Analysis”.

5.1.2 Oil Input Data

Amount of Oil in Largest Container (gal) = If the amount of oil in the largest container is larger than ten percent of total oil, input gallons of oil in largest container from Attachment #1 “Inventory and Spill Prediction Table” of the specific substation SPCC Plan, or as provided by the RE. Otherwise, input “N.A.” for not applicable or leave this cell blank.

Ten Percent of Total Aggregate (gal) = If ten percent of total oil is larger than the amount of oil in largest container, input ten percent of the total aggregate from Attachment #1 “Inventory and Spill Prediction Table” of the SPCC Plan. Otherwise, input “N.A.” for not applicable or leave this cell blank.

5.1.3 Basin Input Data

5.1.3.1 General Input

Average wall slope, h/v = Input the ratio of the horizontal length divided by vertical length of sloped portion surrounding the basin (if applicable). If slopes differ, use the average value of all side slopes. If the wall slope is vertical, enter “0”.

X (ft) = Longer inside basin dimension, based on a rectangular shape, as shown in Figure 5.1

Y (ft) = Shorter inside basin dimension, based on a rectangular shape, as shown in Figure 5.1

L (ft) = Inside width of Weir shown in Figure 5.1

  • L is typically between 1 and 10 ft. For smaller weirs, L typically ranges from 1 to 3 ft.
  • As L increases, the following decrease:

1) Minimum requirement for A (see Section 6.1.3)

2) Required head, E, from “Time of Concentration” (see Section 6.2.4)

5.1.3.2 Elevation View Dimensions

Note: Refer to Figures 4.1 through 4.3 for illustrations of the following variables.

A (in) =Sluice slot height, usually 3” minimum

  • Dependent on required head, E (refer to Section 6.2.4 for definition of E)

B (in) = Skimmer wall height

B′(in) = Height difference between top of perimeter and top of skimmer wall

  • If no perimeter wall exists, enter “0” for B′.

F (ft) = Horizontal distance between weir and skimmer wall

  • F is typically between 1.5 and 10 feet. For construction space requirements, F is usually not less than 18 inches. For small weirs, with the approval of the RE, F may be less than 18 inches, but in no case shall F be less than A+3”.
  • If the weir system is outside of the basin, enter a negative value so the spreadsheet can either add or subtract this volume from the total volume (depending on analysis case).

D (ft) = Weir wall height

  • Measured from top of weir wall to the basin invert at the weir

C′(in)= Freeboard from top of oil to top of weir or skimmer wall, typically 1”, or as provided by the RE

M (ft) = Horizontal distance between weir wall and outlet pipe in the perimeter wall

  • Measured from the inside of the perimeter wall to the upstream side of the weir wall
  • If the weir system is outside of the basin, enter a negative value. The spreadsheet will subtract this volume from the total volume, since this portion does not contain oil.
  • If no back wall exists, enter “0” for M.

5.1.3.3 Depth Measurements

Note: The top of weir in this section is defined as the elevation at the top of the weir wall. Depths d1 to d4 represent depths from the top of weir wall elevation to the measured ground elevation below the top of weir wall elevation. Refer to Figure 5.1 for a plan view of basin depths. For an example of how input depths d1 through d4 are calculated, refer to Figure 5.2 below.

If the ground elevation is flat, enter the same values for all depths (should be equal to the weir wall height).

If only two depths on opposite ends of the basin are measured, enter them into cells d1 and d3. Since an average depth will be taken from d1 and d2 (d12) and from d3 and d4 ( d34), you may also enter the same depth for d2 as d1 and enter the same depth for d4 as d3.

d1=Distance from top of weir elevation to lowest ground elevation in basin

  • d1 =D = Top of weir elevation – Lowest basin elevation

d2= Distance from top of weir elevation to second lowest ground elevation in basin

  • d2 = Top of weir elevation – Second lowest basin elevation

d3= Distance from top of weir elevation to second highest ground elevation in basin

  • d3 = Top of weir elevation – Second highest basin elevation

d4= Distance from top of weir elevation to highest ground elevation in basin

  • d4 = Top of weir elevation – Highest basin elevation
5.2 Sheet 2: Flow-through Input

Note: Sheet 2, “Existing/New SPCC Basin Flow-through Analysis”, determines whether the basin flow capacity meets the storm water flow-through requirements for Cases A and B for existing basins and Case C for new basins.

5.2.1 Table 2. Drainage Parameters

5.2.1.1 Surface Type

Basin Surface Area (sq.ft) = Horizontal projected area of the SPCC containment basin including side slopes

Foundation Area (sq.ft)= The total surface area of foundations within the upstream drainage area

Paved Area (sq.ft)= The total area of paved surface within the drainage area

Gravel Area (sq.ft) = The total area of gravel surface within the drainage area

Soils Area (sq.ft) = The total area of soil surface within the drainage area

5.2.1.2 Runoff Coefficient (C)

Gravel= Input appropriate runoff coefficient, typically 0.75, or as provided by RE.

Soils= Input appropriate soil coefficient, typically 0.50, or as provided by RE.

5.2.2 Time of Concentration

Note: Refer to Section 6.2.3 for time variable definitions.

Tc (minutes) = Input closest time duration, TD CLOSE, computed at the end of Table 4.

  • Tc is set equal to the rainfall duration interval,TD , closest to the corresponding total time, Ttot. Ttot is the rainfall runoff travel time from the most remote part of the substation through the basin and over the outflow weir.
  • For Ttot values less than 5 minutes, set Tc equal to 5 minutes, the minimum duration interval.
  • As Tc increases, the calculated minimum diameter, dmin, and the design storm inflow, Qin des decrease (refer to Section 6.2.5 for definitions ofdmin and Qin des ).

Percent Full (%) = Percent of water already contained in the basin when the design storm begins.

  • The RE should determine the Percent Full based on site specific conditions.
  • 0% means the basin is empty and 100% means the basin is full when rainfall begins.
  • As Percent Full increases, the time to fill the basin, Tfill, and total time, Ttot, decrease.
5.2.3 Table 3. Time Constants for Flow to Basin

Overland Flow Length (ft) = Longest length between ridge and ditch flow

Ditch Flow Length (ft) = Longest total length of ditch flow from beginning of ditch to the basin

SG= Ground slope in feet per foot. SG is calculated by dividing the change in substation ground elevation, ∆ZG, by the corresponding length, LG, which is equal to either Loverland or Lditch.

  • SG = ∆ZG/LG
  • SG varies on the order of 0.005 to 0.03, for both overland flow and ditch flow.
5.2.4 Table 4. Short Duration Rainfall

Blank Space next to Table 4. Title: Enter letter, number (under station bsn no) and station name found on the “Precipitation Depth-Duration-Frequency” Table from the California Department of Water Resources, Short Duration Precipitation Depth-Duration-Frequency Data Table (ref. 1). If no station name is listed, use the station order number.

Storm Frequency (yr) = Input 25-year return period.

De (in) = Input the depth-duration data, for the corresponding return period from ref. 1.

5.2.5 Discharge Outlet

Discharge of Outlet =Select “Ditch”, “Pipe”, or “Pump” from the list box.

  • Ditch: Fluid flows over the overflow weir, out of the weir system, and into a ditch or open area located outside the weir system. No additional input is required. Delete any entered input.
  • Pipe: Fluid flows over the overflow weir, out of the weir system, and through a downstream pipe. Enter all input described below in this section. Delete any input entered in the cells below “Design storm inflow, Qin des”.
  • Pump: Fluid flows over the overflow weir, out of the weir system, and through a downstream pump system. Enter the manufacturer’s pump capacity, Pcap (in gpm), or as provided by RE. Delete all other entered input.

Manning’s roughness coefficient “n” (ref.2)

  • Concrete/Steel:n = 0.012
  • PVC: n = 0.010
  • CMP: n = 0.023

Drop of outlet pipe, a (ft) = Vertical distance between inverts at each end of outlet pipe

Run of Outlet pipe, b (ft) = Horizontal length of outlet pipe

Outlet Pipe Diameter, d (in) = Inner diameter of outlet pipe

Pump Capacity, Pcap(gpm) = Manufacturer’s pump capacity, or pump capacity as provided by RE

6.0 CALCULATED OUTPUT

Note: If a non-integer output (e.g. #NUM, ##, #VALUE!) appears in an output cell, an invalid input was entered. Check to make sure that the correct input values were entered.

6.1 Sheet 1: Oil Storage Output

Note: Sheet 1, “Existing/New SPCC Basin Storage”, calculates the amount of oil to be contained and the allowable oil storage.

6.1.1 Oil Output Data

Oil to be contained (gal) = The maximum of the following:

1)110% of oil in the largest container

2) 10% of total aggregate oil volume

Oil to be contained (cu.ft) = Gallons of oil to be contained divided by 7.48 gallons per cubic foot
6.1.2 Basin Output Data

Note: Refer to Figures 4.1 to 4.3 from Section 4 for illustrations of the following variables.

C (in) = Height from inside bottom of weir to top of perimeter wall

C = A + B + B′

D′ (in) = Height difference between top of perimeter wall and top of weir wall

D′ = C –D

d12(in) = Average basin depth on weir side of basin. Takes the average of depths d1 and d2 and is typically equal to the weir height (see Figures 6.1 to 6.4).

  • d12 = (d1 + d2) / 2or d12 = d1(if only two depths are measured)

d34 (in) = Average basin depth on opposing side of basin. Takes the average of depths d3 and d4

(see Figures 6.1 to 6.4).

  • d34 = (d3 + d4) / 2 ord34 = d3(if only two depths are measured)
6.1.3 Normal Minimum Requirements

Note: The minimum values calculated from the equations below are required for adequate containment.

A (in) = Depth of flow over the weir plus freeboard

  • A = E + C′(refer to Section 6.2.4 for calculation of E)

B (in) = The maximum of the following:

1) The weir wall height from Cases A or

2) For Case B the maximum of the following two conditions are used to maintain a pressure balance between the fluids. The variable Z′, where Z′= (d12-A- C ′)/0.85 + C′, defines the pressure balance. The Z′ and Z′(condition 2) variables apply only to Case B.

a) Condition 1: If Z′ < B-C′, then B = Z′ + C′

b) Condition 2: If Z′ > B-C′, then B = Z′(condition 2) + Creq. For condition 2, a required freeboard, Creq, must be calculated to prevent oil from flowing over the skimmer wall. In addition, a new pressure balance, Z′ (condition 2), must be defined using the calculated required freeboard.

  • Creq = (d12– A - 0.85 x B + 0.85 x C′)/0.15
  • Z′ (condtion 2) = (d12 – A - Creq)/0.85 + Creq

1) B = D

  • MAX

2) MAX a) B = Z′ + C′

b) B = Z′(condition 2) + Creq

F (ft) =One (1) foot plus sluice slot height

  • F = 1’ + A

D (ft) = The maximum of the following:

1) The height of oily water from Case A, How Case A(see Section 6.1.4.1), plus freeboard or

2) The height of oily water from Case B, which equals the maximum of the following two conditions used to maintain a pressure balance between the fluids.

The variable Z′, where Z′ = (d12 – A- C′)/0.85 + C′, defines the pressure balance.

a) Condition 1: If Z′ < B- C′, then D = ( A + C′ ) + ( Z′ - C′ ) x 0.85

b) Condition 2: If Z′ > B- C′, then D = ( A + Creq ) + ( Z′(condition 2) - Creq) x 0.85

1) D = How Case A + C′

  • MAX

2) MAX a) D = ( A + C′ ) + ( Z′ - C′ ) x 0.85

b) D = ( A + Creq ) + ( Z′(condition 2) - Creq ) x 0.85

CASE A / /
CASE B / /

6.1.4 Table 1. Oil Storage Summary

Note: The critical oil storage value has “GOVERNS” printed to the right.

6.1.4.1 Height of Oily Water in the Basin, How

Note: How is the height of oily water at the weir system location.

1) CASE A: Height of oily water equals the weir wall height, D, minus the user defined freeboard, C′

  • How = D – C′

2) CASE B: The height of oily water depends on the pressure balance and whether the oil must be prevented from flowing over the skimmer wall. For condition 1, How is equal to Z′minus the user defined freeboard C′. For condition 2, the pressure balance must be maintained to prevent oil from spilling over the skimmer wall. Thus, How is equal to

Z′(condition 2) minus the calculated required freeboard, Creq.

  • Condition 1: If Z′ < B – C′, then How = Z′– C′
  • Condition 2: If Z′ > B – C′, then How = Z′(condition 2) – Creq

6.1.4.2 Gross Oil Storage Amount

Note: The gross oil storage calculation is based on a prismatically shaped basin with constant side slopes. The basin floor slope, ∆Zo, is calculated by taking the difference between opposing average basin depths, d12 and d34, and is defined as ∆Zo = d12 - d34(refer to Figures 6.1 through 6.4 for illustrations of ∆Zo). For a sloped floor, ∆Zo equals a non-zero value.