Design and application of black water regulating valve
The black water regulating valve is in the severe working condition of solid-liquid-gas three-phase mixing. The causes and hazards of its extremely short service life are analyzed in terms of solid, liquid and gas, and solutions are proposed from several angles to provide reference for future designers.
In the fields of coal to methanol, coal to ethylene, and coal to ammonia, coal-water slurry gasification process promotes the mass production and economy of coal chemical industry. In the coal-water slurry gasification process, in addition to the flash tank as the core device of synthetic conversion, the black water regulating valve in the gasifier and scrubber tower is also an indispensable component. The long-term stable operation of the black water regulating valve affects the continuous production of the whole plant, but under severe working conditions, the service life of the black water regulating valve is extremely short.
1. Analysis of black water working conditions
The black water flash system undertakes the task of fluid and liquid level regulation from the gasifier, scrubber tower and flash tank.
Black water is the associated products produced in the process of coal-water slurry gasification, which needs to be recovered and purified and secondary use.
The typical medium of black water is solid-liquid two-phase flow, in which the solid phase mass concentration can be as high as 3000 mg/L. Along with multiple recovery and settlement, the mass fraction of solid particles is more than 4%, and the particle diameter and shape are not uniform.
The Ca2+, Mg2+, Cl–, S2-, CO2-3, NH+4 and other strong corrosive ions in the liquid-phase flow will cause erosion to the inner wall and parts of the regulating valve. As the black water regulating valve is mostly used for bottom recovery and feeding after the tank, it carries a large amount of liquid itself, and the medium temperature in the process preparation is 200-300℃, which makes the pressure difference between the front and rear of the black water regulating valve larger.
According to the principle of throttling, when the medium flows through the internal shrinkage section of the regulating valve, the pressure does not exceed the saturation vapor pressure at the inlet temperature, the fluid medium will continue to vaporize and form a gas-liquid coexistence phenomenon at the exit of the regulating valve, that is, flash vaporization. As the main role of the black water regulating valve is to reduce pressure regulation of the medium, which will certainly occur flash phenomenon, vaporization of black water fluid volume expansion, and the inflow of black water before the regulating valve formed a solid-liquid-gas three-phase flow.
2. Black water working conditions generated by the hazards
Solid-liquid-gas three-phase flow accompanied by high differential pressure, high flow rate, strong corrosion, strong impact, the working conditions of the black water regulating valve can be described as extremely severe.
2.1 Solid phase
Due to the high solid phase content and particle hardness, under the conditions of high pressure difference and high flow rate, solid particles become the primary difficulty in the design of the regulating valve. First, the high-speed impact of solid particles will hit the fluid path on the obstruction of the forward parts, such as the spool, valve seat, stem, etc.. This part of the regulating function of the parts due to its processing materials, raw materials secondary treatment method, impact resistance structure, resulting in its surface craters, scratches, cracks. These defects caused by the impact of solid particles will erode the parts again with the influence of liquid flashing and cavitation until the parts are broken, bent and fractured.
Secondly, solid particles through the inner wall of the pipe, valve body cavity, due to blocking the flow or pipe turning, valve cavity turning process suddenly change direction, solid particles accompanied by liquid under the action of inertia, impact on the inner wall, resulting in pipe and valve body scratches, scratches, foreign noise. The change of direction will also affect the local media flow rate, resulting in eddy, turbulence, causing the pipeline, valve cavity vibration, noise.
Again, solid particles produce settlement problems. Solid particles are small, very easy to settle, scaling phenomenon, easy to accumulate in the adjustment valve cavity dead center, as well as the spool, valve seat, stem surface, especially for stabilizing the stem, reduce vibration of the stem guide area. To ensure the stability of the stem and guide, the gap between the two will not stay too large, while the stem will move up and down with the valve opening and closing, will inevitably bring solid particles to the gap between the stem and guide, which will lead to the stem and guide jamming, intensifying the wear of the stem and guide surface, and ultimately affect the stabilizing effect of the stem. At the same time, it will also make the spool vibrate under the liquid impact because of the weakening of the stabilizing effect of the guide, and collide with the valve seat, causing secondary damage to the spool and the valve seat. Due to the wear and scratch on the surface of the stem, it will also affect the effect of the upper packing and stem seal of the regulating valve, resulting in serious accidents such as black water leakage and toxic gas escape.
2.2 Liquid phase
As the black water flash system, the regulating valve is facing a high differential pressure conditions, so the black water liquid phase is an important part of the test regulating valve strength. Because of the high pressure differential conditions, in order to reduce the valve opening and closing, set up in the valve cavity for balancing the pressure on the upper and lower sides of the spool and stem balancing area. The liquid under high pressure will impact the balancing area inside the regulating valve cavity, and the flow channel shape will increase the pressure bearing of the spacer in the valve cavity if it can not produce the effect of fluid diversion, steering and deceleration, resulting in cracks and penetration in the valve cavity.
At the same time, the impact of the black water liquid will also directly affect the strength of the valve parts, resulting in vibration, wear, bending of the valve parts, thereby reducing valve stability, adjustment accuracy, safety.
2.3 Gas phase aspects
Black water regulating valve faces high differential pressure conditions, flash and cavitation can not be avoided. Vaporization of the black water fluid volume expansion, the spool surface, seat sealing surface, valve outlet venturi flare extension tube or pipe impact, and even cause parts surface breakage and pipe breakage. At the same time, it will also lead to vibration of the pipeline and downstream process equipment orifice.
Solid-liquid-gas three-phase flow mixing conditions, making the service life of the black water regulating valve is very short, only 1-3 months. The designer’s ability is the key to the service life of the regulating valve, and the following will provide guiding solutions to various problems.
3.1 Type of regulating valve structure
The designer must first determine the type of structure of the regulating valve. Shorten the time of media flow in the regulating valve. Control valve structure is generally divided into straight-through and angle type, see Figure 1.
Figure.1 The structure model of the control valve
In the regulation of black water straight-through type control valve requires two sharp steering, and solid particles are easy to settle in the bottom of the pipe and steering. The angle type with smooth steering and less settling, its structure can well guide the flow of media. When the valve is opened, the medium can flow out of the regulating valve without almost any resistance, minimizing the impact of the medium on the regulating valve.
3.2 Solid particles impact parts
Valve parts are directly exposed to the fluid pathway, the direct impact of solid particles under the action, significantly reducing the service life of the parts, see Figure 2.
Figure.2 Example of impact before optimization
3.2.1 Reinforced parts
The material material and structural dimensions of the part determine its service life. The regulating valve stem is thickened and carbide is sprayed on the critical impact points to enhance the service life of the parts.
3.2.2 Optimize media flow
Diversion of the media flow or optimization of the flow structure of the parts to avoid direct impact on the parts, see Figure 3.
Before the fluid enters the spool, it is guided to form a certain angle with the spool by flow channel bending, which significantly weakens the impact on the spool radially and also reduces vibration to ensure stable valve operation. Valve parts can have a certain angle, curvature, reduce the protrusion, step groove and other structures that may affect fluid flow and increase erosion.
Figure.3 Example of optimized impact
3.3 Solid particle deposition
Fine solid particles tend to accumulate in the cavity of the regulating valve dead center or crust in the spool, valve seat, stem surface, resulting in blocked circulation, see Figure 4.
Figure.4 Example of deposition before optimization
3.3.1 Reduce deposition location
Design the valve cavity and parts into a structure consistent with the flow direction of the fluid, and the valve cavity path to form an overall streamline, and actively reduce the static dead space of the valve cavity, the steps of the parts easy to deposit and other structures. Fluid in the flow process can spontaneously bring solid particles out of the valve cavity inside (see Figure 5), which helps reduce solid particle deposition and improve the flow capacity of the valve cavity structure.
Figure.5 Example of optimized deposition
3.3.2 Evacuation deposition
Under temperature, solute, pressure and other factors, solid particles will adhere to the valve cavity and the surface of the parts, so the valve cavity structure and the surface of the parts need to be set up static evacuation structure. Valve cavity internal easy to deposit location design drain, through the curved, inclined, pit and other structures, the use of gravity to guide the solid particles to the drain to gather, but also the use of fluid flushing deposition location to achieve the role of auxiliary cleaning (see Figure 6).
Figure.6 Example of optimized deposition evacuation
On the part surface, structures such as grooves and ring grooves are used to guide the adhering solid particles to prevent the transitional fit from causing solid particles to seize due to intrusion, see Figure 7.
For the guiding part that needs small fit, solid particles attached to the mating surface cannot be avoided, so you can choose spiral ring groove + vertical groove to hang down the solid particles partially attached to the mating surface, and the solid particles that have already entered the mating surface are discharged by liquid scouring and gravity along the ring groove to reduce the chance of failure jamming. The ring groove and groove shape can be matched with the fluid path to assist in the discharge of solid particles.
Figure.7 Example of optimized part deposition evacuation
3.4 Liquid corrosion
In the design of the black water regulating valve, should fully consider the corrosiveness of the fluid medium and the scouring of solid particles.
For the transport role is greater than the pressure-bearing role of the basic pipeline, can use lower carbon content of corrosion-resistant carbon steel or stainless steel material, which is easy to scour the key parts such as elbows, reducer flange at the use of stainless steel substrate and weld or spraying to strengthen the pipeline scouring and corrosion resistance.
For pressure-bearing role is greater than the regulating role of black water control valve, can use corrosion-resistant, high structural strength of austenitic stainless steel or duplex stainless steel material, which increases the local thickness in the fluid turning, sealing and other major scouring points to ensure the corrosion margin and improve reliability.
3.5 Fluid impact
The main body of the black water medium is fluid, so the shape and structure of the internal flow channel of the valve cavity affects the pressure recovery coefficient of the valve body, which is the key to ensure the stability and safety of the valve body.
Irrational flow channel design (see Figure 8) will make the inner wall part of the bulkhead to bear unnecessary impact, the flow channel internal turning stiff; useless grooves in the transition part will make the fluid in the flow process in a turbulent state, resulting in the valve cavity internal vortex and turbulence, impact spool, valve cavity, causing vibration.
Figure.8 Example of fluid impact and eddy flow before optimization
Referring to the location of solid particle deposition, improving the smoothness of the flow channel so that the liquid flows out of the valve body without obstruction is the focus of the flow channel design. The flow path return and diversion capacity can be increased so that the inner cavity of the valve body can carry the liquid pressure evenly and reduce the pressure on one side of the spool, which helps stabilize the spool (see Figure 9).
Figure.9 Example of optimized liquid shock and vortex
3.6 Coping with cavitation and flash
Black water regulating valve at the valve outlet needs to deal with the high pressure difference brought about by the liquid vaporization and volume expansion caused by the cavitation effect, will produce serious scouring of the valve seat outlet and downstream pipeline. At this point the structural design can play a relatively limited role, can only consider other ways to improve the outlet conditions.
The current feasible method is to install venturi flare at the internal valve seat outlet and extend the venturi flare length to control the cavitation phenomenon in the high strength venturi to protect the downstream piping and equipment.
Since flashing cannot be avoided, the strength of the local structure at the valve seat, spool and venturi can be enhanced by spraying or through-body carbide. In addition, a large diameter buffer tank can be added after the venturi, as well as thickening or overlaying or spraying carbide and other wear-resistant materials in the downstream pipeline.
The impact of high-frequency vibration and noise at the outlet of the black water regulating valve should be considered before the pipeline laying stage, and the pipe support design and anti-vibration measures need to be enhanced.
Black water regulating valve as a representative of the harsh conditions in the design of coal chemical industry, testing the designer’s design level and experience. Designers need to find a balance from the cost, effect, use of the three directions, for the valve “long-term stability” to lay a good foundation.
Author: Fuhao,Wenhao Mu
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.pilgrimpipeline.com)