Flow measurement and control
Flow meters, flow indicators and flow switches: components that measure or show the amount of fluid passing through the line, or signal when the flow stops.
In a hydraulic circuit, speed is determined by flow rate; if a cylinder runs slower than expected, finding the cause without measuring what the pump delivers is guesswork. In troubleshooting and unit testing we decide by measuring the flow rate, and for lines that need continuous monitoring we select the measuring and control component.
Types
Flow indicators
Installed in the line, they show the flow on site; used on cooling water and lubrication lines to see that there is flow.
Flow switches
Give a contact when the flow drops below the set value or stops; used to stop a machine whose cooling water has failed.
Flow meters
Measure the flow rate with a digital or analogue signal; used for testing, recording and control.
How to select
In flow measurement the fluid, the flow range, the pressure and the connection size are determined together. A flow meter made for oil does not read correctly on water, nor one made for water on oil; viscosity affects the measurement.
If continuous measurement is not required, a flow switch is sufficient in most cases: it checks that the cooling water, lubrication or circulation is really running and protects the machine when it stops.
Pump flow rate is displacement multiplied by speed; if the measured flow is clearly below this, the pump is worn or there is a leak in the circuit. We make this comparison as the first step in troubleshooting.
Flow rate by line inside diameter and flow velocity
| Hose / line inside diameter | 1 m/s (suction line) | 3 m/s (return line) | 5 m/s (pressure line) |
|---|---|---|---|
| 1/4" (6.3 mm) | 1.9 L/min | 5.6 L/min | 9.4 L/min |
| 3/8" (9.5 mm) | 4.3 L/min | 12.8 L/min | 21.3 L/min |
| 1/2" (12.7 mm) | 7.6 L/min | 22.8 L/min | 38.0 L/min |
| 5/8" (15.9 mm) | 11.9 L/min | 35.7 L/min | 59.6 L/min |
| 3/4" (19.0 mm) | 17.0 L/min | 51.0 L/min | 85.1 L/min |
| 1" (25.4 mm) | 30.4 L/min | 91.2 L/min | 152.0 L/min |
| 1 1/4" (31.8 mm) | 47.7 L/min | 143.0 L/min | 238.3 L/min |
Calculation: flow rate = flow velocity × cross-sectional area. The velocities are common design values: 0.5-1.5 m/s in the suction line, 2-4 m/s in the return line, 3-6 m/s in the pressure line. The flow meter and the flow switch are selected according to this line diameter and flow rate.
Frequently asked questions
My hydraulic cylinder has slowed down, is the pump the cause?
It may be, but an internal leak in the circuit gives the same symptom. Measuring the pump flow rate tells the two possibilities apart.
What is the difference between a flow switch and a flow meter?
A flow switch only gives a contact at the set value; a flow meter measures the amount of flow continuously.
Can a flow meter be used with any fluid?
No. A flow meter is selected according to the type and viscosity of the fluid; the same instrument reads differently on oil and on water.
How can I estimate pump flow rate without measuring it?
Theoretical flow rate is displacement multiplied by speed. If the measured flow is clearly below this, the pump is worn or there is an internal leak in the circuit.
Does line diameter affect flow rate?
Yes. In a narrow line the flow velocity rises, and pressure loss and heating increase; if the suction line is too narrow, the pump cavitates. The line diameter is selected according to the required flow rate and the recommended flow velocity.
Where is a flow switch used?
On cooling water, lubrication and circulation lines, to stop the machine or give an alarm when the flow stops.
Stock, sizes and part numbers
We select and supply the products in this group according to the application. Send us your requirement, the size and the operating conditions on WhatsApp and we will recommend the right product.