This standard provides guidelines for the use of Manometers in process plants. Main keywords for this article are Manometers Design Requirement in Process Plant. References Manometers. Definitions Manometers. Tube Manometers. Liquid Manometers.
References Manometers
American Petroleum Institute (API)
RP 550/551 Installation of Refinery Instruments and Control Systems
Definitions Manometers
For the purpose of understanding this standard, the following definitions apply.
Manometer.
A gauge for measuring pressure or a pressure difference between two fluid chambers. U tube manometer consists of two legs, each containing a liquid of known specific gravity.
Meniscus.
The concave or convex surface, caused by surface tension, at the top of a liquid column, as in a manometer tube.
Pulsation Dampener.
A device installed in a gas or liquid piping system to smooth out fluctuations due to pulsating flow and/or pressure.
Manometers Design Requirement in Process Plant
- Manometer is based on the principle of difference in hydrostatic pressure on two liquid columns. Pressure applied to the surface in one column causes an elevation in the liquid surface of the other column. The amount of elevation is read from a linear scale and is converted into pressure by an appropriate multiplier.
- All the three types of pressures (positive, negative and differential) can be measured by manometers.
- Manometers provide accurate means of pressure measurement and are used as calibration standards for shop and field instruments.
- The range of U tube manometers is from 6 to 140 inches. It depends on the length of the tube and the specific gravity of the fluid.
- The reading is accurate upto 0.2 mm.
- Excessive use with the passage of time does not change accuracy or calibration.
Tube Manometers
- The tubes for manometers used as laboratory standards shall be greater than 0.375 inch in diameter.
- The inside diameter of the tube for pressure upto 400 psig shall be 1/4Â inch. For higher pressures upto 2000 psig, the inside diameter shall be 1/8 inch.
- The tube shall be made of pyrex glass.
- Inclined tube manometer shall be used for greater sensitivity and higher accuracy.
- Plain glass tubes shall not be used.
Liquid Manometers
The following points shall be taken into consideration before selecting manometer fluid.
- The specific gravity of the fluid shall be known and shall not change appreciably with changes in temperature.
- The fluid shall be compatible to process media and chemically inert.
- It shall produce a clean and visible interface.
- The fluid shall not flash or freeze at its operating temperature.
- The fluid shall not coat the tube and shall not be corrosive to steel, copper or aluminum.
- For safety of personnel mercury shall not be used.
Operation Manometers
- To eliminate error caused by meniscus effects, the reading shall be taken at the center of the tube.
- Special traps shall be used to catch blown liquid in case of overpressure.
- Manometer shall be mounted close to the point of measurement.
- For measuring unsteady pressure, pulsation dampeners shall be used.
- Installation shall conform to API 550/551.