Dry Piston Pumps: A New Option for Vacuum Pumps

Nov. 4, 2005
Vacuum pumps — most of us have been exposed to these aircraft components. For aircraft gyros, there have been two main choices for vacuum pumps — dry and wet. Both types of pumps use the same technology of rotary vanes to provide suction (or pressure for pressure-type systems). But there is a new kid on the vacuum pump block: a dry piston pump manufactured by Sigma Tek. We will take a look at how it works and address some maintenance issues mechanics need to be aware of.

Vacuum pumps — most of us have been exposed to these aircraft components. For aircraft gyros, there have been two main choices for vacuum pumps — dry and wet. Both types of pumps use the same technology of rotary vanes to provide suction (or pressure for pressure-type systems). But there is a new kid on the vacuum pump block: a dry piston pump manufactured by Sigma Tek. This is a PMA direct replacement pump for many piston engine applications. The pump is a totally different design from current vacuum pumps. We will take a look at how it works and address some maintenance issues mechanics need to be aware of.

How it Works

When you first see a Sigma Tek vacuum pump, you will notice right away that it doesn’t resemble the rotary vane pumps it is designed to replace. Not only does it look different, it operates totally differently than rotary pumps.

The basic design of the piston pump is simple. There are a total of 16 umbrella valves, eight on either side of the piston. The piston is operated by a crank mechanism that is driven by the engine. The design of the crank is such that it drives the piston straight up and down in the cylinder, thus avoiding side loading on the cylinder wall. As the piston pushes toward one side of the pump, four of the eight valves shut with the pressure produced, while the other four open allowing air to go through and out the pressure side of the pump. On the other side of the piston, the opposite is happening — four of the valves close while the other four open up allowing air to enter from the vacuum side of the pump into the pump chamber. Once the piston reaches full travel, it reverses direction. Now, the air that was drawn into the pump is expelled out the pump pressure side while the other side is drawn into a vacuum. So, in each half of the full cycle, air is being drawn in and expelled, creating a constant vacuum and pressure during operation.

Pressure Degradation

The piston pump design doesn’t lend itself to catastrophic failure. The design of the valves is such that several valves can fail and still not significantly affect the pump’s performance. However, failure of an excessive number of valves will result in a degradation of pressure. Monitoring pressure output is a good way to monitor pump life. For example, if you are normally at 5.2 inches pressure at cruise you may see that go down to 4 inches if some valves aren’t working properly. This is a good indication that the system and/or the pump needs to be looked at.

Before Installation

Although the design of the piston air pump makes it less susceptible to failure by foreign object ingestion, it is still necessary to clean out the vacuum system prior to installation. Ensure all lines are free of debris. This is especially critical if you have a rotary pump failure since there will be carbon dust and debris in the lines. This is also a good time to check the condition of all flexible lines and replace them as necessary.

Installation

The Sigma Tek pump installs on the same mounting pad as the original rotary vane pump. One thing that mechanics should keep in mind is that the design of the pump is such that it is bi-directional. That means that the vacuum port and the pressure port will remain the same regardless of whether it is driven clockwise or counter-clockwise. There is no need to know engine rotation or worry about stocking different pumps for different rotating drives.

The pump can be mounted in any orientation. The mechanic can mount it in the orientation that will allow for the best clearance and easiest plumbing. Sigma Tek shared that for the most part, the pump will either be installed vertically where the piston head is vertical up, or horizontally where the piston head is to the left-hand side toward the accessory case when you are looking toward the front of the aircraft. It boils down to installing it in a way that allows for the easiest installation. Each shipped pump comes with a flange gasket.

This gasket should be replaced whenever a new pump is installed If rotating the pump does not solve the issue of clearance, there is a solution that can help. Sigma Tek manufactures a ¼-inch spacer that can be installed between the gasket and pump flange. If you do need to install a spacer, only this spacer can be used.

Another consideration during installation is to install the lines properly. The vacuum and pressure ports are clearly marked on the pump. When installing fittings on the pump ensure that only low loss (thin wall) are used (instead of AN-type fittings). In addition, if you need to install a 90-degree fitting, you need to install one from Sigma Tek. This is because the typical 90-degree fitting is a little too long from the thread centerline to the end of the fitting. This causes the elbow to hit the pump housing when being installed. The recommended fittings are a little shorter in this dimension allowing for an unobstructed installation.

Instrument Flutter

Although the design of the piston pump is such that it is constantly creating vacuum and pressure, there is a very slight delay at the top of the stoke where there is no output. In typical operation, this slight pulsation has no effect on the vacuum gauge. However some vacuum gauges that have a 2-inch line may experience an oscillation of the needle depending on whether the gauge has an orifice in the inlet port. This can be resolved by installing a snubber in the line going to the gauge. This snubber is available from Sigma Tek.

Engine Cleaning

Mechanics used to working on rotary vane pumps are aware of the need to cover the vacuum pump during engine cleaning to avoid contaminating the pump. Although the piston pump is not as susceptible to damage from intrusion, Sigma Tek recommends protecting the pump mount area to avoid any intrusion of debris. As an added precaution, it suggests installing a fitting on the pump with at least 12 inches of hose attached pointed downward. That helps keep moisture and contaminants from entering the pump.

When it comes to creating a vacuum (or pressure), this new pump does the same job as its rotary counterparts — it just has a different way of doing it.