Mechanical Engineering

Valve which is meant to siphon off a portion of the flow from another system.

A heavy-duty gas turbine employs compressor bleed valves to protect the axial compressor during start-up and shutdown against stalling and surging conditions. These valves are typically only open during acceleration to rated speed and deceleration from rated speed. Newer heavy-duty gas turbine with complex combustion system arrangements designed to limit emission of harmful gases also employ the compressor bleed valves during very light loading to limit the amount of air entering the combustor.

Compressor bleed valves discharge into the gas turbine exhaust duct. Compressor bleed valves are generally closed during normal operation (with the previous exception) since "dumping" axial compressor discharge into the exhaust reduces efficiency (but it's MANDATORY during start-up and acceleration to protect the compressor!). Compressor bleed valves are generally either fully open or fully closed; they are not modulated.

Heavy-duty gas turbines also employ inlet bleed heat valves to recirculate a portion of the axial compressor discharge back to the inlet of the axial compressor for a variety of functions. One application uses them to heat the inlet air to prevent icing on the compressor inlet (appropriately called anti-icing protection); another application uses inlet bleed heat to protect the axial compressor when the IGVs (axial compressor Inlet Guide Vanes) are closed below the "normal" operating conditions ("normal" being before complicated combustion systems designed to limit exhaust emissions were invented).

Inlet bleed heat control valves are modulated to control the amount of air recirculated back to the axial compressor inlet. Recirculating axial compressor discharge back to the inlet is also inefficient, but is generally limited to approximately 5% of rated air flow through the compressor--and necessary to protect the compressor at low IGV angles. Inlet bleed heat valves are generally closed before the unit reaches rated load (except for anti-icing protection) and should be closed during rated power output (Base Load) operation (except for anti-icing protection).

forming. These techniques could include methods such as explosive forming, electromagnetic forming and so

forth. All these techniques involve imparting a high kinetic energy to the work piece by accelerating it to a

highly velocity, before it is made to hit the appropriate die or made to undergo the process of plastic the

formation.

I must also tell you that although this method seems very new, it was discovered more than 100 years ago and

metals were being worked upon using HVF techniques as early as the 1930s. Due to the nature of the

operations it would also be appropriate to name this technique as highly velocity deformation rather than high

velocity formation.

One main advantage of using HVF techniques is that very complex shaped parts can be formed in a single

operation, rather than carrying out a series of operations to achieve the same results via conventional forming

techniques.

A wide variety of operations which are conventionally carried out using traditional methods can be done using

highly velocity techniques, and these processes include those of extrusion, die-forging, punching, joining and

so forth. The list of materials which can be formed using high velocity methods contains a wide variety of

materials including magnesium, aluminium, zirconium, stainless steel, alloys steel etc.

Advantages of HVF

Highly velocity forming techniques have several advantages over conventional forming techniques, apart from

reducing the number of processes required in manufacturing.

The strain distribution is much more uniform in a single operation of HVF as compared to conventional forming

techniques. This results in making it easy to produce complex shapes without inducing unnecessary strains in

the material.

Since the basic principle of a highly velocity formation technique is quite opposite to that of a conventional

forming technique, the tools and other equipment used in the process has quite lightweight compact relatively

speaking.

High velocity techniques can be used to improvise upon various joining methods such as say for example

fusion welding. Impact welding using highly velocity techniques can produce much better results than the

conventional fusion welding techniques