Design of diffuser (reducing stage assembly) for superheater and reheater start up vent silencer in super critical once through boilers

In supercritical boiler Startup Vent diffuser is to manage the instant release of huge amounts high pressure of steam to low pressure into atmosphere while cutting of the noise produced while venting the steam in different temperature and pressure conditions. The upstream pressure of startup vent is around 81 kg/sq.cm. so for reducing pressure from 81 kg/cm2 to 3kg/cm2 with constant flow one diffusers (reducing stage assy.) will introduce which will diffuse the pressure. Startup Vent diffuser is a design a reducing stage assembly (called a multistage diffuser) along with study of hydrodynamic parameter like Beta ratio, Mach number, and numerical calculation of orifice plate thickness, size of orifice hole & Bending stress evaluation, reducer etc.


Introduction
When boiler is warming up initially the turbine will be matched only a specified temperature and pressure. Till such time the steam produced will be let out to atmosphere. For this purpose, startup vent is required. Its other purpose is for safety valve floating, that is to check the safety valve set pressure. This system includes start up line, regulating and isolating valves, diffuser and silencer, to abate noise.

Problem identification
In the startup vent line, the silencer inlet pressure has to be maintained within 3 Kg/sq cm for suit to start up vent silencer.
For the super critical boilers, the upstream pressure of startup vent will be at around 81 kg/sq.cm. So for reducing pressure from 81 kg/cm2 to 3kg/cm2 with constant flow one diffusers (reducing stage assy.) will introduce which will diffuse the pressure.

Remedies
In order to eliminate the problem, we have many remedies such as introducing with multiple pressure reducing valve which leads higher cost, operation & maintence cost & also increasing plant cost. If we design a reducing stage assay, it can replace the pressure reducing valve. It has simple design with very less cost & weight as compared to valve.

Methodology of study
Parts of reducing stage assy. are 1. Conical nozzle 2. Orifice meter

Conical nozzle
The application of a conical nozzle was very common in early rocket engines. The principal appeal of the conical nozzle is that it is easy to manufacture and it has the flexibility of converting an existing design to lower or higher area ratio without much redesign.
The geometry of converging nozzles affects the conditions at which critical-subcritical flow transition occurs. The objective of this is to develop guidelines to identify the optimum nozzle geometry that maximizes critical pressure ratio while minimizing pressure drop across the nozzle. It was determined that a smaller diverging angle and absence of an elongated throat resulted in a higher critical pressure ratio. Length of converging and diverging sections of nozzles did not have as much of an impact on nozzle performance as the throat diameter and shape of converging and diverging sections.

Orifice meter
The simplest and most common device for measuring flow rate in a pipe is an orifice meter. An orifice meter is a device with a hole in it, which measures how fast a fluid is flowing, by recording the pressure decrease across the hole. The two most important factors that influence the reading of an orifice meter are the size of the orifice and the diameter of the pipe which it is fitted into. orifice plates are most commonly used to measure flow rates in pipes, when the fluid is single-phase (rather than being a mixture of gases and liquids, or of liquids and solids) and well-mixed, the flow is continuous rather than pulsating, the fluid occupies the entire pipe (precluding silt or trapped gas), the flow.

Beta Ratio (β)
The orifice plate has a typical bore diameter that ranges from 30% to 75% of the inside diameter of the pipe work in which it is installed. A beta ratio of 0.4 signifies that the orifice bore diameter is 40% of the pipe inside diameter. The Beta ratio (Ratio of orifice whole diameter to Orifice Plate Diameter) is kept around 0.4.

Mach No
Mach number is an important quantity to compare the speed of any object with a speed of sound. It is a parameter to denote the speed of a flying object in air medium. In fluid dynamics, it has very vast use. Mach number is heavily studied to have a proper idea on the motion of aero planes and rockets.
 When the Mach number is less than unity, the flow is called a sub-sonic flow.  When the Mach number is equal to unity, the flow is called a sonic flow.  When the Mach number is between 1 and 6, the flow is called a supersonic flow. Mach number (M or Ma) is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a boundary to the local speed of sound. M=u/c M is the local Mach number, u is the local flow velocity with respect to the boundaries (either internal, such as an object immersed in the flow, or external, like a channel), and c is the speed of sound in the medium, which in air varies with the square root of the thermodynamic temperature.

Reynolds no
The Reynolds number (Re) helps predict flow patterns in different fluid flow situations. At low Reynolds numbers, flows tend to be dominated by laminar (sheet-like) flow, while at high Reynolds numbers flows tend to be turbulent.
The Reynolds number (Re) helps predict flow patterns in different fluid flow situations. Types of flow are classified into 2 types: laminar flow and turbulent flow. Reynolds number helps us to determine whether the flow is laminar or turbulent. At high Reynolds numbers, the flow tends to be turbulent.  Each stage has one reducer and orifice plate. Selection of reducers and orifice plates depends on requirement and material availability.  Flow parameters are maintained in each stage which has been showed.  Calculated value of the MACH number comes in subsonic range in all stages, which means better noise reduction.  The ratio of orifice plate hole diameter to orifice plate diameter is kept around 0.4    So shear thickness is less than the plate thickness hence design is OK

Check for Mach No.
Mach number calculated has to be less than 0.

Stage II
The same above procedure is applicable for reducing 40.5 Kg/cm 2 to 20.25 Kg/cm 2 with Assumed Pressure drop= 0.5

Stage III
The same above procedure is applicable for reducing 20.5 Kg/cm 2 to 10.125 Kg/cm 2 with Assumed Pressure drop= 0.5

Stage IV
The same above procedure is applicable for reducing 10.125 Kg/cm 2 to 5.062 Kg/cm 2 with Assumed Pressure drop= 0.5

Stage V
The same above procedure is applicable for reducing 5.062 Kg/cm 2 to 2.53 Kg/cm 2 with Assumed Pressure drop= 0.5

Figure 5
Five stage diffuser assembly

Results and discussion
Thereby the problems associated high pressure drop in startup vent line in thermal power plants have been eliminated by designing equipment like diffuser (reducing stage assay) within the existing constraints.

Conclusion
This paper proposes a feature-adaptive method called designing of reducing stage assembly which is simple design and also easily fitted at boiler start up vent system. We can conclude that the developed reducing stage assembly can be used for pressure drop with constant steam flow and the efficiency also significant increased.