Question 1

Types of PD Flow Meters

Accepted Answer

PD Flow Meters are also known as positive displacement flow meters. It is mainly composed of measuring body and measuring part. The metering body consists of a shell and a movable body (rotor of the rotameter). The core part is the movable body, which is the moving part in the measurement process. Each measuring cavity formed by the movable body and the casing is a standard volume unit, which is used to measure the volume of the fluid to be measured.

Extended reading: Fluid flow meter types

The measuring part is actually a rotational speed or counting sensor. Magnetic material is embedded in the pinion extending out of one of the screws on the screw flowmeter. When the screw rotates, the sensor can identify the rotational speed of the screw through the magnetic effect. And convert it into a standard pulse signal or current signal. In this way, the flow can be directly displayed by the secondary instrument or collected and analyzed by the computer.

There are many types of positive displacement flow meters. They can be classified according to different principles. Usually, according to the structure of the measuring element, it is divided into rotor type, scraper type, rotary piston type, reciprocating piston type and diaphragm type.

Among them, the rotor type is the most commonly used. The common gear type, screw type, waist wheel type and egg wheel type flowmeter belong to the rotor type.

Extended reading: Positive Displacement Flow meter Technology

Question 2

PD Flow Meters Working Principle

Accepted Answer

The interior of the positive displacement flowmeter is designed with a space that constitutes a certain volume. The rotor inside the flowmeter rotates under the action of the pressure of the fluid flowing from the inflow port.

With the rotation of the rotor, the fluid flows from the inflow port to the discharge port. At this time, during the rotation of the rotor, a certain volume space is formed between the rotor and the flowmeter housing. The fluid fills this space with the rotation of the rotor. It is continuously sent to the discharge port.

If the volume of the space is designed, the number of rotations of the rotor is measured. The volume given by the space can be obtained, thereby obtaining the volume of fluid flowing through.

Extended reading: Liquid mass flow controller

Question 3

What are the applications of positive displacement flow meter?

Accepted Answer

This fixed-displacement flow measurement method for positive displacement flowmeters can be traced back to the 18th century. It entered the stage of widespread commercial application in the 1930s.

In recent years, the sales of PD flow meters in some industrialized countries have exceeded 20%. In my country, it also accounts for about 20%. It is mainly used in the metering of petroleum-based media.

But because of its precise metering characteristics. In recent years, it has been rapidly expanded to chemical, food, medical and other sectors to accurately measure the total amount and flow of expensive media.

In the measurement of petroleum, an international standard (ISO/DIS 2714) for measurement systems using positive displacement flowmeters has been developed. Moreover, positive displacement flowmeters with high precision, long-term performance retention and high repeatability are also used as standard instruments for industrial flow measurement, calibrating and calibrating industrial instruments.

In short, due to the high accuracy of positive displacement flowmeters, they are often used to measure the flow of relatively expensive media. High viscosity media can be measured. Therefore, it is generally used for the measurement of high-viscosity liquids in petroleum, chemical, metallurgical and other industries.

Extended reading: How to choose a high viscosity flow meter?

Of course it is also suitable for measuring low viscosity liquid media. In addition, positive displacement flowmeters can also measure gases. At the same time, the installation of positive displacement flowmeter does not require the front and rear straight pipe sections.

Extended Reading: Liquid Turbine Flow Meter

Question 4

sanitary positive displacement flow meter

Accepted Answer

The Sanitary Positive Displacement Flow Meter is made of stainless steel and is quickly connected by Tri-clamp. Can be used to measure viscous liquids in the food industry. Such as sweeteners, edible oil, palm oil, honey, fat, tallow, fat, syrup, molasses, etc.

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Question 5

Flow range and accuracy

Accepted Answer

The manufacturer generally specifies the flow range according to the type of the measured medium (mainly different viscosity), use characteristics (continuous use or intermittent use) and measurement accuracy.

In order to maintain good performance and long service life of the flowmeter. It is recommended to select 80% of the maximum flow rate of the flow meter when the maximum flow rate is used continuously.

If the manufacturer does not clearly specify the flow range according to the type of medium and the characteristics of use. The maximum flow rate for intermittent use can be 100% of the upper flow limit.

When used continuously, the maximum flow rate of medium viscosity liquid is 80% of the upper flow rate of the flowmeter. The maximum flow rate of low-viscosity and high-viscosity liquids is 50% to 60% of the upper flow rate of the flowmeter.

The fundamental error of most positive displacement flow meters is ±0.5%. The basic error of the higher precision flowmeter is ±(0.1%-0.2%). The oval gear flowmeter can reach ±0.05%. The basic error of the lower precision flowmeter (such as the elastic scraper flowmeter) is ±(1.0%-1.5%).

The accuracy of a positive displacement flowmeter is the fundamental error obtained under laboratory conditions. In actual use, field conditions often deviate from laboratory conditions, resulting in additional errors. The actual error should be the sum of the basic error and the additional error. When selecting and using, measures should be taken according to the on-site situation to ensure the measurement accuracy.

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Question 6

pressure loss

Accepted Answer

Positive displacement flowmeters rely on the fluid to push the measuring fittings, so the pressure loss is high. The pressure loss of a positive displacement flowmeter is larger than that of other types of flowmeters of the same diameter and flow. The pressure loss of general liquids is 20 to 100 kPa.

When measuring liquids with high vapor pressure, excessive pressure drop will cause cavitation. If cavitation exists for a long time, it will damage the flowmeter accessories. Some flowmeters that allow short-term overflow to 120% of the upper limit of measurement should pay more attention to this problem .

Extended reading: Orifice Plate Flow Meter

Question 7

Fluid Corrosive

Accepted Answer

Fluid corrosiveness is a major factor in determining flowmeter material.

For various petroleum products, cast steel and cast iron are used.
For slightly corrosive chemical liquids and cold water, it is made of copper alloy.
Made of stainless steel for pure water, high temperature water, crude oil, asphalt, high temperature liquids, chemical liquids, food or food raw materials.

The corrosion resistance of positive displacement flowmeters is generally not strong. In the food and biopharmaceutical industries, due to hygiene requirements, flow meters are frequently sterilized. Fittings that come into contact with fluids must be made of stainless steel and other hygienic materials. They must be easily disassembled.

Question 8

Liquid viscosity

Accepted Answer

Generally speaking, the viscosity of various gases is similar. It has no effect on the performance of the flowmeter. The viscosity of liquids is quite different.

In order to adapt to the liquid with high viscosity, the positive displacement flowmeter has a structure with a large gap.

Although the volume-second flowmeter is affected by viscosity, it is less affected than differential pressure and float turbine flowmeters. Viscosity generally affects the measurement error, pressure loss and flow range of positive displacement flowmeters.

Extended reading: Ultrasonic Flow Meters Types & Technical Guide

Question 9

pressure and temperature

Accepted Answer

When the flowmeter is used at higher temperatures, the maximum working pressure rating must be reduced. Sudden valve opening and closing can create water hammer effect. Shock force may exceed operating pressure. Shock force may also cause false readings. Buffer tank may be installed if necessary.

Temperature will not only affect the compressive strength of the flowmeter, but also affect the measurement accuracy. Reduced structural clearance can jam moving parts. Therefore, when used at higher temperatures, a dimensional clearance should be reserved to compensate.

Especially when different materials are used in combination, pay more attention to the difference in thermal expansion coefficient.

Changes in temperature also change the viscosity of the liquid, causing flow changes. It can be corrected by automatic temperature compensation, and there must be a suitable preheating time before use, and observe whether it can operate normally.

Extended reading: Heating Oil Flowmeters

Question 10

compressibility

Accepted Answer

Usually the compressibility of liquids is negligible, however, compressibility should not be neglected when measuring oil with high accuracy.

For example, when the pressure of heavy oil rises from 0.5 MPa to 6 MPa, the volume is compressed by 0.45%. LPG is more compressible.

Gases are highly compressible, and their volume decreases at low pressures in proportion to pressure increases. Most positive displacement flow meters are used for low pressure conditions and can be directly converted.

However, under high pressure conditions, the volume reduction is not proportional to the pressure increase. The gas compressibility factor should be considered.

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