What Is An Inline Process Refractometer?

Inline sensing head
Inline sensing head
Inline process refractometers are used in the food processing, beverage, pulp and paper processing, chemical, power, and primary metal industries. Inline process refractometers save labor costs, streamline the manufacturing process, and improve quality by detecting inconsistencies in a product early in the production phase. They continuously monitor, control and report the on Degrees Brix, Percent Solids, Dissolved Solids, SGU, and refractive index of a brand variety of solutions.

Inline process refractometers are made up of three main components:
Control console
Control console
  1. The inline sensing head, which is inserted to a pipe and is exposed the the product for direct measurement. The sensor is connected by a cable to the second main component, the control console.
  2. The control console, which houses the display, electronics, power supply and output connections. Both the sensor and the control module are designed for high accuracy and precise control. Their housing and cabling are often specialized for use in varying extreme conditions and very demanding environments.
  3. The process adapter, which connects the sensor mechanically to the process piping. Adapters are configured for the pipe size and application they are used on and are available in many configurations. Some of the more common adapter options are:
    • Lined - For extremely corrosive applications.
      Mounting adapters
      Mounting adapters
    • Inline - For most applications with pipeline sizes less than 2”.
    • Isolation - For critical process lines that cannot normally be shut down.
    • Sanitary - For applications on a sanitary pipeline.
    • Spool - For most applications with pipeline sizes greater than 2”.
    • Vessel - For applications where the sensing head is to be mounted directly onto a vessel or vacuum pan.
    • Weld-in - For economical mounting.

The Three Major Causes of Refractometer Trouble in Black Liquor Recovery Boilers

Pulp and paper mill
Pulp and paper mill.
The three major causes of refractometer trouble or failure in black liquor recovery boilers are:
  1. Loss of cooling water and its effect on the sensing head.
  2. Lack of reliability of the prism wash.
  3. Condensation in the sensing head.
These may not apply to all refractometers due to differences in construction and circuitry.

Cooling Water Loss

It is of vital importance that the loss of cooling water be detected. This may be done through a temperature sensing element or flow monitor which shuts down the refractometer involved.

Damage to the sensing element of a refractometer does not occur instantaneously, but it is essential that the system detect abnormal temperatures due to cooling water loss, flow blockage, etc., and that the cooling water be promptly restored.

The individual refractometer manufacturer’s instruction and maintenance manuals shall be consulted with reference to: potential damage to the sensing element; identification of a damaged element; how and when to replace a damaged element.

Prism Wash

The time interval between prism washes may vary with the black liquor composition. It is recommended that the minimum wash period be 7-10 seconds of wash every 20 minutes. Short duration washes at more frequent intervals are more effective than long washes at long intervals. Ideally, steam pressure for prism washing should be 35 psig above the black liquor pressure, plus the pressure required to open the protective check valve.

Awareness must be maintained of the effect of changes to the prism wash programming variables. Various refractometer systems have the capability to adjust: condensate drain time, steam on time, recovery time and interval between wash time. It may be possible to configure the system to have the total time that both refractometers are in their wash cycle represent a significant percentage of operating time. If one refractometer is out of service for repairs and the remaining refractometer is in prism wash, black liquor solids are not being monitored. Prism wash should be minimized to that needed to maintain the system.

If high pressure steam is used, it may abrade the prism. If only high pressure steam is available, a reducing valve shall be used.

The refractometer prism must have a clear polished optical surface, and if it becomes abraded, it must be replaced.

If the prism wash system has not operated properly and the prism becomes coated, it must be removed and properly cleaned.

Condensation in Sensing Head

Condensate may build up in the refractometer sensing head and if this occurs, the instrument operation will be erratic.

The procedure for determining this condition and for the elimination of excessive moisture in the sensing head is not the same for all refractometers. The manufacturer’s instruction and maintenance manuals shall be consulted and followed carefully.

Reprinted from "Recommended Good Practice: Safe Firing of Black Liquor in Black Liquor Recovery Boilers" courtesy of the Black Liquor Recovery Boiler Advisory Committee.

Image by AlexiusHoratius [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons