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What is full Flow and a partial flow condition?

Maximum flow occurs when a pipe is operating at full capacity. Fig. 1 (a) depicts the full flow situation. In other terms, full flow occurs when a pipe is flowing to its entire depth. A pipe has a partial flow when it is not operating at full capacity or depth. Fig. 1b depicts the partial flow in the pipe, when the flow is half its full depth.

Full Flow Condition Partial Flow condition

full flow situation
Fig.1(a)
partial flow in the pipe
Fig.1(b)

What is Flow rate?

The volume of water flowing from the pipe per hour is known as the flow rate. It is simply computed by multiplying the cross-sectional area of the pipe by the velocity of the stream flowing through it.

Flow rate = Area of the pipe (A)* flow velocity (V)

Read More: Top 9 Water Measurement Essentials Explained

Effect of full Flow and Partial flow on the flow meter?

To compute the flow rate, the flow meter uses the same equation.

In the case of full flow, the pipe's area is just its cross-sectional area. The area of the pipe with full flow is shown in Fig. 2.

Area of the pipe in full flow condition = π*D^2/4  for flow meter diameter "D"

Fig. 2:

water in full flow in a pipe
Full flow
Cross Section Of The Pipe
Cross Section Of The Pipe

Flow rate = A*V.

As, entire cross-sectional area of the pipe cannot be used to calculate the flow rate, while the pipe is running in half. In partial flow situations, just the highlighted section of fig. 3 will be used to compute the flow rate. D = D/2 will be the depth to calculate the area.

Fig 3:

Cross Section Of The Pipe
Cross Section Of The Pipe
Partial water flow in a pipe

However, while determining the flow rate, the flow metre will take into account the entire pipe area. As a result, even if the pipe is only half full, the entire area of the pipe will be evaluated. The flow rate value is erroneous as a result of this. The ultrasonic flowmeter thinks the water is full and calculates the discharge volume even when there is only 'partial flow.' As a result, ultrasonic flow metres record a higher flow than the actual flow, implying that the discharge through the pipe is exaggerated.To understand the implications of partial flow in a flow metre, we will now compute the flow rate for both partial and full flow instances. Let us look at an example to better understand this scenario:

Problem: Consider a pipe having a flow velocity of 10 m/hr and the pipe has a diameter of 2 m, we will try to evaluate the flow rate for both half flow and full flow conditions.

1) For full flow:

D= 1m

A =   =  =3.14 m2

Velocity V= 10m/hr

The flow rate = 10 * 3.14 = 31.4 m3/hr

2) Partial Flow:

As mentioned earlier the flow mentioned earlier for the partial flow area will be:

D1= D/2 = 1m

A =   =  =1.57 m2

Flow rate = Area * Velocity

= 1.57 * 10 = 15.7 m3/hr

The discharge was reported to be 15.7 m3/hr, when there is partial flow, however the flow metre calculates the flow rate to be 31.4 m3/hr, which is twice the actual flow rate. This results in a 50% exaggerated value. It is critical to maintain full flow in the pipe at all times to avoid measurement inaccuracies. Otherwise, there will be a measuring irregularity if there is a partial flow. For proper measurement, the pipes should have a full flow.

Read More: Science Behind Flow Measurement

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