Optimization of Electric Angle Type Needle Valves for Alumina Digestion Flash Tanks

Part I :  R & D context

 

Digestion process control is paramount in alumina refining

Performance of control valves directly affects production

In digestion electric angle type needle control valves are installed between flash tanks

 

Harsh operating conditions:

Mixed Solid-Liquid-Gas medium

Abrasive solids in the slurry cause erosion  

High temperature and pressure

Large pressure drops

Part 2   Tube digestion process

High temperature, pressure tube digestion technology is widely used for alumina refining

In the tube, turbulence of bauxite slurry flow allows for efficient heat transfer and short digestion time

Hard to dissolve diasporic bauxite requires 9 pre-heating stages followed by 8 flashing stages before discharging in the dilution tank.

Valve operating conditions

Installation position: between flash tanks in pressure digestion system

Function: Control fluid pressure and flow rate

T = 140℃ - 280℃ and P = PN4.0MPa  - 160MPa

Process fluid: slurry, aluminate  solution, Na2O 220 g/l

olid-liquid-gas 3-phase medium

Flow rate = 285 m3/h - 350m³/h and Density = 1500kg/m3

Pressure drop: △P = 2.6 - 0.18MPa

Control range：30 : 1     Leakage：ANSI Ⅳ

Harsh operating conditions require valves to have special structural design of durable material and with good flow characteristics for accurate process control

Part 3   Structural improvement

1、Structural improvement of valve body

Streamlining angle valve body:

We improved the structure on the base of figure b, and designed the new type as figure a

Disadvantage of figure b:

valve disc and seat are placed at valve’s throttle, more erosion  of seat and disc when flow areas  change.

 Disadvantage of figure b design: Baffle and localized protection device in the valve body give some protection of the stem, but transferring the washing force  to valve sleeve and disc will shorten the lifetime of sleeve and disc.

 Structure b can easily cause eddy fields in the valve body, impacting on control accuracy and destroying valve body and baffle.

 

Resulting in blocked valve outlet and ultimately the angle needle valve does not work  and the quickly rising pressure in the flash tank will cause an accident.

2.Structural improvement of valve stem

   New design of valve stem protection sleeve:

Control valve’s  stem erodes easily. Without a protection device and special hardening treatment, valve stem will seriously wear out and even break in a few days

1. Example of washed out valve stem
    
   

The valve stem of after being improved

New bonnet structure with a protective sleeve on the stem

Abrasion -resistant alloy plated on the surface.

Form of stem reduces stress and prevents wear

                                        3 D Model of the improved stem protection design

3、Improvement in material of valve disc

The disc (inner part of valve) is easily washed out by the fluid.

High strength hard composite material-WCB+HF is designed to replace the WCB

hardness of  WCB  < HB137

hardness of HF > HRC70.

Improved erosion–resistance performance due to hardness of valve disc

Easy Maintenance

The valve disc and valve seat usually have to be tested in operating conditions.

Figure a: Angle control valve can be disconnected from the whole allowing testing of disc and seat
In operation, seat and disc are visible and quick-wear parts can be easily replaced.

Figure b:  Whole valve has to be disassembled for testing disc and seat, and maintenance is complicated.

Part 4 Benefits

                        

Valve lifetime doubled

 Less maintenance cost of digestion piping due to longer lifetime of the valve

Less repair of throttle orifice and jet. Down time caused by orifice failure, erosion and replacement are avoided after installation of the angle needle valve.

 

Schematic feeding device for flash system

Structural improvements have doubled the lifetime and enhanced the control performance of  angle type needle valves for alumina digestion flash tanks.

As the valve Installation, digestion’s operating cost is largely reduced .