Product(s):	HAMMER
	Version(s):	V8i, CONNECT Edition
	Area:	Layout and Data Input

Problem

What do the "Weir Coefficient" and "Weir Length" fields represent, in the properties of a Surge Tank?

Solution

The weir length and weir coefficient are used together to simulate overflow over the top of the weir if the HGL exceeds the maximum elevation of the surge tank during a transient simulation in HAMMER.

The Weir Length represents the perimeter of the top of the surge tank, over which water will spill out when overflow occurs. For example if the surge tank has a box shape at the top with a length of 5 feet and a width of 4 feet, the weir length could be set to 18 feet. In other words, if overflow occurs, water would spill over both 5 foot side and both 4 foot sides at the same time, for a total effective length of 18 feet.

The weir coefficient is the standard weir coefficient used in the Weir equation, which is used to calculate the overflow along with the weir length. HAMMER uses the standard weir equation:

Q = CLH^1.5

Where:

Q = flow (cfs/cms)
L = weir length (ft/m)
H = head (ft/m)  - this is the overflow elevation - hydraulic grade minus maximum elevation.
C = weir coefficient (typical values: ~3.0 US units, ~1.8 SI units)

To see the overflow rate, enter a number in the "Report Period" field in the surge tank properties, re-compute the transient simulation and look for a table for the tank near the end of the Transient Analysis Detailed Report (located under Reports > Transient Analysis Reports). In it you will see a column for "Spll-Rate" which is the overflow calculated from the weir equation in the event that the tank overflows. See more on Report Period in the link further below.

Note: 

• If your tank is sealed and cannot overflow, consider the "variable area" option for the "Section" property, with a tiny area above the top of the tank (so as to still allow surcharging; imagine a small "chimney" or "straw" rising above the top).
• When using the variable area section type, the weir length and coefficient fields are not used, and HAMMER assumes that if the water surface elevation rises above the top of the cross section curve, all excess inflow becomes overflow.

See Also

How to graph extended transient results such as gas volume for hydropneumatic tanks, pump or turbine speed, air valve extended data, etc.

How can I see the amount of inflow or outflow occuring for a particular surge tank during the transient simulation in HAMMER?

Modeling Reference - Surge Tanks