Needle Valve basic function :
Needle Valve are one of the oldest and most basic mechanical components in fluid handling systems. Although there are many variations in the types and designs of valves, most perform one of two basic functions of valve :
Isolation :The valve is function to open or closed; its function is work is to turn the flow on or off by connecting, it thus isolating it from the rest of the system.
Regulation :The valve is function to regulates the rate of flow through the system and it usual is neither fully open nor fully closed.
How Needle Valve Work
- Isolation
Isolation (shut-off) Valve which Screw actuation allows the valve to opened gradual and the thread stem is requires three or four turns of the handle to open and close the valve. For exceptionally high pressures general above 10.000 Psig (690 bar) with a special variation of needle valves has been design. Although similar to other needle valves for this type of valves contain unique design features with materials specially selected for high pressure operation. - Regulation
Regulating and metering valves have function to design adjust the rate of flow. They usual have a long fine tapered stem tip and a fine pitch stem thread to open in five to ten turns. This type of Valve are not suggested use for isolation service and are general designed they cannot used function as shut off (see Fig. 1)
Needle Valve Function Flow Control Regulating
The next type of valve function is flow control. The purpose of this type of valve is to regulate flow through a system whether it be simple regulation or fine metering. Although the primary design function is to control flow, on the other hand some valves also provide on-off control.
Needle valve has multiple functions. In this case, it is being used to regulate flow. Flow will be determined by orifice size, stem type, and stem position which can be controlled by turning the handle. This valve may also be referred to as a globe type due to the flow path.
Valve Function Flow Control
Regulated Fig 1
Metering Fig 1
If we put it into a simple circuit, it allows us to control the motor speed by limiting flow.
Needle valve may be available with different stem geometries which may include ball, vee or plastic tips that will allow for shut-off functionality. Since the primary function of a needle valve is to regulate flow, if shut-off is required, one of these alternative stem tips should be considered. As a caution, if shut-off is attempted on a valve with a needle stem geometry, damage may occur. Fig 2 show Type of Stem of Needle Valve.
Fig 2 : Stem Needle Valve Design
Pressure Temperature Ratings Needle Valve
Pressure and temperature Valve ratings specify the maximum and minimum temperatures and pressures which the valve can be operate safe. These ratings are based on the properties of materials and used in pressure containing parts cause those of other materials that contact the system fluid. On design require and on performance tests. Design basis for our process and instrument valves, explains how pressure temperature ratings are determined. Its lists national codes and standards relating to valve ratings.
Selection Process System Parameters
These are some of the system parameters that should be considered during the selection process. It is important to consider the entire range of the parameters your valve may encounter.
- Pressure
- Temperature
- Media
- Flow
- Environment
- Energy Management
Pressure
Although we have already covered pressure, the concept of pressure differential is more appropriately discuss here with flow.
Delta P or pressure differential is the difference between the upstream and downstream pressure across a valve. Differential pressure is a natural aspect of a valves construction. A pressure drop will occur as system media is moving through a valve. For instance, a valve with a globe pattern will have a higher pressure drop across the seat than will a ball valve of similar size where the flow is straight through. The higher the pressure differential, the more or greater impact system pressure may have on wear of internal components.
What is illustrated through computational fluid dynamics or CFD, is a depiction of pressure drop across two different valve types. At the same flow rate, to achieve the same downstream pressure, shown in light blue, a higher pressure is needed on the inlet side of the globe pattern valve shown on the left. This is illustrated by a more extreme color gradient. The ball valve, or straight through pattern on the right will show less pressure drop, as indicated by the colors.
System Parameters
The first set of parameters we would like to look at is pressure and temperature. The reason why we put these two parameters together is that they effect each other.
The pressure a valve will see must be consider from various standpoints so we should understand the following:
System Maximum Pressure which is the maximum pressure this valve could possibly see.
Operating Pressure which is the normal operating conditions.
Another system condition that must be consider is temperature. Like pressure limitations, every valve has a maximum and minimum design temperature that should not be exceeded.
Strength of Materials – when the temperature elevates, the strength of materials decline. With valves, this usually means that as temperature goes up, the pressure rating goes down.
Sealing Materials – In many cases the pressure or temperature limiting component in a valve is the seal material.
