Dimensioning of an Expansion vessels Multifunction for domestic hot water use

The dimensioning of the expansion vessels for domestic water systems requires the same formula of the heating systems.

Dimensioning of an expansion vessel as water hammer arrester

The calculation of the storage volume against water hammer is essential to protect the system. Two formulas are provided for two different cases: first, in which the system pressure (P) is known; second, in which the flow (Q) of the system is known.
Knowing the pipe diameter, (D) crossing flow and pressure values as in the table below then could be obtained the linear capacity value (C).
In the case in which the flow is known/round up, if the value given is not equal to the one given by the table, then make a rounding up to obtain the value:

Qa Approximate flow (m3/h), the approximate value in the table immediatly above the.

Table of calculation of the linear capacity C

D (“) 1/2”3/4”1”1,5“2”2,5“3”3,5” 4”
Speed (m/s)1.121.19 1.381.481.471.51.581.62
Q (m3/h)
P (bar)Minimum linear capacity of the vessel (lt./m)
120.00370.0046 0.0104 0.02530.04180.06710.09410.12980.1660

Given l = pipe length (m), the formula to calculate the adequate storage volume (V) is as follows:

  1. Pressure known: V=C·l
  2. Flow known: V=C·l ·(Q/Qa) 2

The adequate expansion vessel will be the one with capacity immediately above V.
CD-ROM ‘Varem strong points’, available on demand, contains a calculation application

Example of calculation with flow known

l = 180 mPipe length
D= 1”Pipe diameter
Q = 2.5 m3/h--> Qa = 3 m3/hFlow known (Q); approximated flow (Qa)
C = 0.1271Linear capacity (see table)

V = 0.1271 l/m · 180 m · (2.5 m3/h / 3 m3/h)2 = 15.89 l ————> Extravarem LC R1 18 lt.