Rotational Valve FSU

Project Introduction

General Electric Oil and Gas is a company that focuses on the drilling, production and storage of natural gas as well as industrial power generation, refining and petrochemicals. Before the storage and transportation of natural gas, the gas must be run through a gas compressor. The gas compressor is similar to a pump. The compressor acts to increase the pressure on the gas and to transport the gas from one place to another. By increasing the pressure on the gas, the volume is reduced which in turn increases the density allowing more gas to be transported at minimal volume.

The compressor’s valve is the controlling factor for scheduled compressor maintenance downtime. Extending the time between shutdowns, or producing more flow between shutdowns is desired. Furthermore, reducing the time it takes to replace the valve will also be beneficial.

The most common accepted valve in today’s compressors is the plate valve (fig.1).The valve uses the pressure within the cylinder to open and allow gas to flow through them. The valve acts in an open and close linear fashion. Though the valve is reliable long term and cost efficient, the valve adds unwanted energy to the gas in the form of heat due to inefficient flow, which also reduces the flow rate. In an industry where time is money, a low volumetric flow rate is undesired. Another valve used today is the poppet valve (fig.2). The poppet valve operates much like an automobile engine’s valve works (fig.3). Though the valve used in the compressor looks more complicated than the automobile valve, the operation is much the same. When the pressure on the flat side of the valve, or on the pressure side in the case of the compressor valve, the valve springs compress allowing gas or air to pass. Though the flow rate is satisfactory, the direction of flow is still a non linear and turbulent.

The focus of the project is to design a valve that operates in a rotational manner. To clarify, rather than sliding open and closed linearly as the plate valve’s design, the designed valve must rotate in a clockwise manner to allow flow to pass then rotate back to close the valve. It is desired that the valve is able to be used as an intake and exhaust valve.

Plate Valves

A plate valve, for this example a ported plate valve (fig.4 & 5), is composed of a guard, seat, moving element, damping plate, springs, and guides. When not under pressure, the moving elements are held tightly against the stop plates by way of springs. When the valve undergoes pressure, the moving elements are displaced along the guides as the springs compress. When this happens, passageways are exposed allowing gas to pass. Once the pressure drops on the face of the cylinder, the moving element return to the original position closing the passageways.

Though reliable in terms of wear, the valve design results in inefficient flow. When the passages are opened, the do not line up.In order for the gas to displace from one of the valve to the other, the gas must take multiple ninety-degree turns.This adds unwanted energy to the gas in the form of heat. Furthermore,this indirect path reduces the flow rate able to be achieved by the valve

Poppet Valves

Poppet valves (fig.6) are composed of more parts than the plate valve. In the poppet valve itself are many poppets, hence the name poppet valve. Though more moving parts, poppet valves achieve a higher flow rate with less power loss (fig.7).

When the valve is exposed to the design pressure, the poppets w thin the valve are displaced by the compressing of the springs. This allows for the flow of gas through the valve. Once the pressure is reduced, the poppets return to their original position, closing the valve.

Current Valve Comparison

Concept Generation

It is possible to achieve the desired outcome of a rotational compressor valve with several methods. To properly satisfy the costumers’ needs, certain parameters must be followed by each valve design. If any of the parameters are not followed, the project will not be considered properly completed. The projects’ parameters set forth by the sponsor are as follows:

• The valve must operate in a rotational manner and obtain laminar flow • The valve is to operate at pressures between 30 psi and 600 psi • The materials composing the valve must be able to withstand long exposures to gas and temperatures approaching 350F • The design must be easily modified to fit all current gas compressors used by G.E. • The valve is to be easily replaced • The project, including research, design, materials and machining, should fall within $2000 • The cost to mass produce the valve should be competitive with the poppet and plate valve • The valve is to outperform the volumetric flow rate of the current plate and poppet valve

With the above parameters, we were able to conceptualize five possible concepts. These concepts will be weighed in terms of: reliability, cost, efficiency, volumetric flow rate, and feasibility of installation. The heaviest weighing factor is whether the designed valve can be easily installed to the existing compressors used now days without modification to the compressor itself. If modifications must be added, the valves performance must justify the complications of installation. But overall, each factor will be analyzed thoroughly and weighed.

Concept 1:Mechanical/Pressure Activation

The rotational pressure valve can be activated using both pressure and mechanical components. One way to achieve this is by fitting pressure devices in the wall of the cylinder. These devices will have a surface exposed in the cylinder that will “give”, or be displaced, under pressure or vacuum. When the surfaces are displaced, the motion will be used to move an external linkage. This linkage will be externally connected to the valve to cause the rotation of the valve.

For intake purposes, the unit drilled into the wall of the cylinder will be activated by vacuum. The reason for this is that the intake valve is to be activated during the down stroke. When the vacuum displaces the exposed unit, an external linkage is translated to rotate the valve. Once the pressure in the cylinder is no longer negative, the exposed unit returns to the original position, which in turn closes the valve.

Once the cylinder has completed the intake stroke, the pressurization and exhaust of the gas is followed. To achieve this, there will be another unit that displaces under pressure. When a designed positive pressure is achieved, the face of the unit is displaced. Just as the intake process, a mechanical linkage is translated opening the rotational exhaust valve. Once the pressure is relieved, the exposed unit is returned to the original position by way of a spring. This then in turn closes the exhaust valve.

Concept 2: Pressure Actuation

The current valves operate when there is a large pressure difference across the valve. The current designs, the poppet and plate valves, use this pressure difference to open and close the valve in a linear manner. As the compressor goes through its compression cycle, the vacuum created by the intake stroke creates this pressure difference, causing the pressurized intake gas to push the valve open. When an adequate amount of gas has flowed into the cylinder, the pressure on both sides of the valve reach equilibrium and the valve is closed by spring force.

To use the pressure differences found in the compressor cycle to actuate a rotational valve, the gas pressure must be used to create a force tangential to the center of the valve. This force creates a rotational movement that then opens the valve.

Pressure actuation is the cheapest and simplest method of valve actuation. It requires no extra systems to be adapted to the compressor. It also has the fewest moving parts and would therefore be the most reliable solution. The problems with pressure actuation are that it is difficult to design and construct the valve so that it properly opens and closes from the pressure difference. Also, it is limited to actuation by pressure and spring force, so its performance cannot be fine tuned very highly. Therefore, it could suffer a small performance disadvantage when compared to other more complex actuation methods

Concept 3: Mechanically Timed Electronic Actuation

Electronic actuation is by far the most open method. Valve rotation can be caused by linear solenoid or rotational motor and can be controlled by anything from a basic analog system run off a distributor to a microcontroller that controls everything digitally. Using electronic sources to open and close the valve allows the valve design to be much simpler because it does not need to have some internal actuation device that could disrupt the gas flow. Also, the timing of the valve can be set to optimize gas flow without needing some built up pressure difference across the valve.

The first electronic concept idea is to use an automotive-style distributor to time the valve motion. The distributor would be rotated by the crankshaft of the compressor and would deliver power to rotational solenoids to open and close the valve. This idea would need some external power source to power the solenoids. Because the distributor is connected directly to the crankshaft, any source of valve mistiming would be eliminated. Also, the valve timing could be fine tuned by adjusting the distributor design to optimize flow by changing the duration of electrical contact inside the distributor. This design is more complex and expensive than the existing valve design but allows much more efficient flow. The added complexity could cause reliability problems, especially due to the fact that a distributor failure could disable half of the compressor.

Concept 4: Digitally Controlled Electronic Actuation

Another method of valve actuation is to use a microprocessor to control solenoids to open and close the valves. The microprocessor could take in the compressor speed and pressure using sensors and then vary the valve timing to achieve the most efficient gas flow. Allowing the microcontroller to continuously vary the valve timing would give this design the highest performance out of any of the designs. The power for the solenoids would come from an outside source, such as the compressor generator or another source. This design is also very complex, which could lead to reliability problems. Sensor failure or error could throw the system off the correct timing, which would make the compressor less effective. Also this design would probably be very expensive due to the high cost of sensors and microprocessors.

Concept 5: Mechanical Actuation

The third method of valve actuation is through some purely mechanical system. Ideally, the mechanical actuation would be directly linked to the crankshaft of the compressor. This would assure that the valve motion would always be directly connected to the compressor’s operating speed and therefore the valve timing could not be off of the compressor cycle. This method could use two different types of mechanical systems, a cam driven system and a linkage system. Either system can be run directly off the crankshaft. The mechanical linkage could use a geared rocker link to rotate the valve so that it rotates back and forth to open and close. Unlike the electric designs, this idea does not need an outside power source. Also, but changing the geometry of the linkage, the valve motion can be changed to flow gas efficiently. The linkage would need to be built very well to handle the high operating speeds. The large forces caused by this high compressor speed could potentially cause failure. The compressor would also have to be extensively modified to install the linkage system.

Conclusion

All of the concepts were created and compared to decide on what is the most effective concept. Although each design had its strengths and weaknesses, it is important to narrow the scope of the project down to one design concept early on in order to focus all attention to one idea. In order to decide which idea was going to be focused on going forward, each was given a score of 1 – 5 on five different weighted performance aspects. These aspects are listed below

Concept Selection Factors

1. Reliability – Reliability is one of the most important factors in valve design. The current plate valve is expected to last one year, so any new valve should be expected to at least match this lifespan. Any downtime of a compressor costs the customer money, so designing a less reliable valve is out of the question. Each design was scored in expected reliability by estimating the different sources of potential failure.

2. Cost – The cost is a very important to the valve design. A cost effective valve design already exists, so the new design must be able to compete with the plate valve on both a performance level and financial level. The cost rating takes into account both the expected cost of the project, which must stay under budget, and the expected product cost once it enters production.

3. Ease of Construction – Ease of construction takes into account how realistic it would be to implement each design given our limited source of resources. 4. Ease of Installation – This factor describes the relative simplicity or complexity of implementing a design on existing compressors. It is important that the new valve design be as interchangeable as possible with the current design.

5. Flow Rate – The current design’s main area of inadequacy is that it restricts gas flow into and out of the compressor. It is important that the new design perform substantially better.

Decision Matrix Factors

Concept Selection

The scores of each design can be seen above in the decision matrix (fig. 12). Although it did not score as well in flow rate and ease of construction, the full rotation pressure actuation had the highest overall score. This is primarily because it is by far the simplest design. The design’s simplicity allows it to be the most reliable and cost effective, while still allowing fairly efficient gas flow rate. The pressure actuation also requires no additional parts to be used, making it the most interchangeable design and the easiest to install.

References