This article is about Level Instruments Design Requirements in Process Industry, Specifications for Level Instruments, Indicators, Differential Pressure Type, Displacement Type and Float Type Level Design, Level Switches Design Requirement (Displacer and Float Type), Gauge Glasses Design Requirements, Magnetic Level Gauges Design Requirements, Radioactive Type Tank Level Instruments.
Level Instruments Design Requirements in Process Industry
General Requirements
- Level instruments shall be directly connected to vessels. Switches used for shutdown service shall not be installed on level bridles.
- Side/side connections for level instruments shall normally be avoided because of nozzle alignment problems.
- Differential pressure type level instruments shall normally be furnished for pressure vessels and tanks unless process conditions make such types undesirable. Flange mounted differential type without block valve may be used where process conditions make this necessary. Flange ratings shall be in accordance with the vessel specifications.
- In general, a differential pressure transmitter shall be selected for level measurement. For interface level measurements, a capacitance type transmitter shall be used.
- External displacer type level transmitters and controllers shall normally be furnished for local level control applications. Instrument cases shall be rotatable with respect to vessel connections. Connections on the instruments shall generally be in accordance with the piping or vessel specifications.
- Materials of construction for level instrument cages or bodies and trim shall be suitable for the process fluid, the temperature and pressure conditions, and shall conform to the vessel
or piping material specifications. For lined vessels, the materials of construction shall be selected for the fluid and service conditions. - Where process conditions dictate, other types of level instruments, capacitance, nuclear, sonic, ultra-sonic, or magnetic float type shall be considered.
- All level instruments on boilers shall be in accordance with the requirements of the ASME Boiler Code.
- Gauge glasses shall be used for local level indication and shall be sized to cover operating and shutdown level range.
- Gauge glasses shall be armored reflex type for all services except the following, where armored transparent type (without illuminators) shall be used:
a. Interface between liquids
b. Distillates below 25 degrees API gravity and all crude residuals
c. Propane and lighter fluids
d. Liquids containing solid material which may coat the flutes of a reflex glass
- Magnetic level gauge units using local indicators shall be considered where applicable.
Specifications for Level Instruments
- Indicators
Level transmitters involved in local control loops shall be furnished with a meter or gauge which indicates the signal output on a 0-100 linear scale. - Differential Pressure Type
a. Differential pressure type level instruments shall have a continuously adjustable range and a suppression or elevation mechanism.
b. Body material shall be compatible to process fluids unless manufacturer’s standard product is constructed of superior material.
c. For the flange-mounted type, the rating shall conform to the specification of the process equipment.
d. External differential pressure type level instruments shall be furnished for pressure vessels in level ranges which exceed 1220 mm, unless process conditions make such types undesirable, and may be furnished for shorter ranges, where accessibility makes them preferable. Flange mounted differential type or alternatively internal displacer type, may be used where process conditions or accessibility make these preferable, regardless of range. For vessels that contain viscous materials, consideration shall be given to flange mounted differential pressure transmitters with isolation valves. For vessel containing slurries or waxy oils, the extended diaphragm type (diaphragm flushed with the inside wall of the vessel) shall be used. Flange mounted differential type shall have manufacturer’s standard size flanges; flange ratings shall be in accordance with the vessel specifications.
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Displacement Type and Float Type Level Design
a. Rotatable head construction is required for external displacers.
b. External displacer type instruments shall not be used in service where the fluid in chamber will boil.
c. The pressure rating shall conform to the specification of the process equipment.
d. Materials shall be stainless steel 316 for the float and carbon steels for the chamber, unless services require different materials, or manufacturer’s standard product is constructed of superior material.
e. Displacer level transmitters shall be used up to 48 inch (1200 mm) in length. For liquid liquid interface measurement, displacer level transmitter shall be used up to 60 in (1500) mm). Vessel connection shall be minimum 2 inch flanged.
f. Radiation fins shall be fitted to instruments in service where the temperature is above 200ºC. An extension bonnet shall be provided to instruments in service where the temperature is below -10ºC.
g. Bottom drain plug shall be NPT 3/4 in. for side and topside connection of external type displacers.
h. Displacement devices shall not be used where accumulation of solids can build up on displacer surface.
i. Internal displacer shall have stilling well where agitation is present in a tank or long length required.
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Level Switches Design Requirement (Displacer and Float Type)
a. The pressure rating shall conform to the specification of the vessel.
b. The electrical contacts shall be the snap-acting single-pole double-throw type. Minimum ratings of 10 A at 120 VAC and 5A at 28 VDC. Microswitches shall be environmentally sealed.
c. Immersed material shall be stainless steel 316, unless services require a different material, or manufacturer’s standard materials are superior.
d. Electric contacts shall be hermetically sealed type.
e. Level switches shall not be employed on process service. Switching function on process service shall be derived from transmitter signals,
f. Local mounted instruments shall meet the relevant electrical area classification.
g. When tankage located adjacent a process unit is operated by an operating department other than the department of the adjacent unit, level alarms shall be provided to sound in the control rooms of both departments.
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Gauge Glasses Design Requirements
Gauge glasses shall be used as local level indicators; however, due consideration shall be given to the properties of the liquid and the likelihood of it attacking the glasses:
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- The reflex type preferably shall be used in all clean liquid services.
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- The transparent type shall be used in the following cases:
a. For liquid-liquid interface service
b. For opaque liquid service
c. When a protective shield to the glass is required such as steam above 20 Bar g, caustic solution above 15 percent, etc
d. When rear-illumination is required
e. When liquid purging is required
- Specifications for gauge glasses shall be based on the following criteria:
a. The pressure rating shall conform to the equipment specification.
b. Reflex type gauge glass columns shall have the minimum pressure rating of 69 bar (1000 psig) at 315 deg C (600 deg F).
c. The gauge chamber shall be of carbon steel and the trim for gauge valves shall be of stainless steel to 420, unless services require different materials
d. The connections between the chamber and the gauge valve should be as follows:i. Chamber side: welded
ii. Gauge valve side: screwede. The gauge valve should be the offset-angle type ball check with union bonnet, ¾ inch NPT male union at valve seat and gauge connection, and with built-in velocity check. The drain valve shall be 1/2-in. gate type.
f. For steam service where the pressure is higher than 10 Bar, a transparent gauge with a mica shield shall be used.
g. For corrosive services, such as hydrofluoric acid and strong alkaline, a protective shield, such as plastic coated glass, shall be used.
h. For cold services where the temperature is below 0 deg C, a non-frosting level gauge shall be used.
i. For hot services, and where necessary to prevent the fluid from congealing, a gauge glass with a steam jacket shall be used.
j. For special applications, such as corrosive or high-pressure service, the magnetic follower type shall be used.
k. An illuminator shall not be provided, unless specified.
l. A gauge glass unit length of approximately 11 7/8 in. (300mm) visible length shall be used. No more than five units may be used in one column. When two or more columns are required to cover a longer range, the visible portion of the gauge glasses shall overlap at least 1 in. (25 mm).
m. Above 400 deg F (205 deg C) operating fluid temperature, a single gauge glass column shall not contain more than three units nor exceed 48 inches (1200 mm) between gauge glass column connections.
n. Tubular glass type shall not be used.
Magnetic Level Gauges Design Requirements
Magnetic level gauges shall be used as local level indicators in clean and non-interface (single-phase) liquids, and where transparent or reflex gauges can not be used. Magnetic level gauges shall confirm to the following requirements:
Radioactive Type Tank Level Instruments
Radioactive type level instruments shall be used for solid slurry or liquid with high solid content measurement.
- Level Measurement – Low Temperature and Cryogenic Service
a. In cryogenic services, differential pressure instruments shall be furnished.
b. When using differential pressure instruments, the principle of self-purging shall be used. The instrument shall be mounted above the vessel/column tapping.
c. In exceptional circumstances seal fluids may be used and the instrument mounted below the tapping.
d. For temperatures above -50 deg C, heat tracing of instrument impulse lines shall be used on all control and alarm loops to ensure that gas legs are maintained. This does not apply where the fluid will remain liquid at ambient temperature and line pressure.
e. Where process conditions dictate, differential pressure transmitters with vapor or gas blowback shall be employed for level measurement.
f. For measurement of level in low temperature services, displacement type transmitters may be used. Chamber material must suit process conditions. Extended bonnets are to be specified according to manufacturer’s recommendations.
g. Tank-gauges on cryogenic storage tanks may be of the motor operated, surface seeking type with digital remote read-out facilities. Each tank gauge shall be provided with isolating ball valve and calibration facilities. The top of the ball valve may be used as reference point. The sensor shall be capable of removal through the valve.
h. Tank-gauges on cryogenic storage tanks may be of the motor operated, surface seeking type with digital remote read-out facilities. Each tank gauge shall be provided with isolating ball valve and calibration facilities. The top of the ball valve may be used as reference point. The sensor shall be capable of removal through the valve.
