BS9251: 2014 - Square law graduation graph
BS 9251: 2014, Annex C.5 ‘Maximum flow demand calculations’ require the tank size to be calculated using the most favourable demand point which is then intersected with the pump curve to find the maximum flow demand for the system which is often referred to as Qmax. By multiplying the Qmax by the system minimum duration time you have the required effective capacity of the tank.
It a very simple method to find the Qmax is if you have graph paper with the x-axis (flow) scale graduated with an exponent scale of 2. This is commonly known as square law graduation graph or paper

Isometric Graph Paper
BS 9251: 2014, Annex C.5 ‘Maximum flow demand calculations’ require the tank size to be calculated using the most favourable demand point which is then intersected with the pump curve to find the maximum flow demand for the system which is often referred to as Qmax. By multiplying the Qmax by the system minimum duration time you have the required effective capacity of the tank.
It a very simple method to find the Qmax is if you have graph paper with the x-axis (flow) scale graduated with an exponent scale of 2. This is commonly known as square law graduation graph or paper
