Usage:
General:
This pneumatic speed setting assembly is used to change the speed setting of SG or PSG governors by means of an air signal, either from a pneumatic controller-as in a process control system-or from a pressure regulator for simple remote speed adjustment. This assembly can be furnished on any SG governor not equipped with a speed adjusting motor, and on any PSG governor except one with two position speed droop, with bi-metal type temperature compensation, or with a speed adjusting motor. When this speed setting-assembly is used, an external speed adjusting shaft is not supplied and the cover is machined for mounting the assembly.
The direct type speed setting assembly increases governor speed setting as the air pressure signal increases. The assembly responds to an air pressure change as small as .05 psi. While the most commonly used assembly receives an air pressure signal of 3 to 15 psi, assemblies using signals in other ranges are available (e.g., 6 - 30 psi, 9 - 15 psi, 3 - 20 psi, 5 - 60 psi). The speed range obtainable varies from about 10 percent to 80 percent of the high speed setting; speed can be set accurately only at one end of the speed setting range. Contact us for help in selecting the correct assembly for a particular application.
Hysteresis in terms of rpm increases as the speed range increases. (A hysteresis loop is a plot of the speeds obtained at various control signal pressures; one portion is recorded as speed setting signals are being increased, the other portion as the signals are being decreased.) In a governor using a speed setting assembly with a speed range of 2850 to 3350 rpm, the hysteresis in rpm is less than one-half of one percent of high speed setting; with a range of 2370 to 3370 rpm, the hysteresis in rpm is about three-fourths of one percent of high speed setting.
Operation:
The pneumatic speed setting assembly consists essentially of a diaphragm, housing, oil reservoir, adjusting screws and a rod extending down through the cover to the governor floating lever. The air signal pressure acts on one side of the diaphragm and causes it to move against the force of an opposing spring. Movement of the diaphragm is transmitted by the diaphragm rod to the governor floating lever. Motion of the floating lever increases or decreases the speeder spring force. The change in speeder spring force represents the change in speed setting.
To prevent hunting which would occur because of air compressibility if the air pressure were supplied directly to the diaphragm, the space above the diaphragm is filled with oil and is connected through an orifice with an oil reservoir. The air pressure signal is applied to the top surface of the oil in the reservoir and the pressure signal is transmitted by the oil to the diaphragm.
Installation:
The pneumatic speed setting assembly cover has two tapped holes for the oil reservoir; selection of the proper tapped hole for the oil reservoir depends on whether the governor is mounted vertically or horizontally on the engine. The tapped hole facing upward is used for the oil reservoir, and a plug is used in the other hole.
The space above the diaphragm and the space in the oil reservoir must be filled with oil so that no air is trapped above the diaphragm. It is easier to accomplish this by filling this space with oil while the governor is vertical.
If the governor is to be mounted horizontally on the engine, assemble the oil reservoir so that it will face upward when on the engine. Remove the plug from the other tapped hole in the cover; hold the governor upright and fill the space above the diaphragm with engine or turbine oil. Replace the plug and install the governor horizontally on the engine. Fill the space in the oil reservoir with oil.
If the governor is mounted vertically on the engine, remove the oil reservoir after the governor has been installed and fill the space above the diaphragm with engine oil. Replace the reservoir and fill it with oil.
Figure 2
Overhaul:
When the pneumatic speed setting assembly is disassembled during overhaul of the governor, care should be taken to avoid damaging the diaphragm. If cracks have begun to form in the diaphragm, it should be replaced. Note that the set screw (4) must be loosened before spring seat (9) can be removed. (The numbers in parentheses correspond to the identifying numbers in Figure 2.)
Before reassembling diaphragm (24), coat all surfaces lightly with Molykote Z lubricant (Alpha-Molykote Corp., Stamford, Conn.) or equivalent. Be careful to avoid wrinkling or twisting the diaphragm during assembly, or premature failures may result. Lubricate O-rings with petrolatum to prevent cutting during assembly.
When the speed setting assembly is assembled on the governor case, be careful that the small end of the diaphragm rod (A in Figure 2) is inserted into the hole (A, Figure 2) in the upper side of the governor floating lever.
Adjustment:
Make the speed droop adjustment first.
Next, proceed to adjust the maximum speed setting with maximum air pressure applied. This adjustment must be made with the engine in operation. Back out the governor high speed stop screw. Loosen set screw (4) and adjust spring seat (9) to obtain the correct maximum speed. Turn in the high speed stop screw until it contacts the speed adjusting lever and lock the stop screw in this position. Tighten set screw (4).
To set the minimum speed, reduce the air pressure below the minimum value and turn adjusting screw (20) until the correct minimum speed is obtained.
The above instructions are for accurate setting of maximum speed at maximum air pressure; consequently, at minimum air pressure, the minimum engine speed will vary somewhat due to the tolerance range on governor parts. This is true because only maximum speed or minimum speed, but not both, can be set accurately, unless governor parts are very closely matched.
If it is desired that minimum speed be the more accurate of the two settings, set the air pressure at the minimum value and back out adjusting screw (20) until it is no longer in contact with the upper end of diaphragm cup assembly (25). With the engine running, loosen set screw (4) and adjust spring seat (9) to obtain the correct minimum speed. Turn adjusting screw (20) in until it just contacts diaphragm cup assembly (25). Tighten set screw (4). Finally, increase the air pressure above the maximum value and adjust the high speed stop screw to obtain the desired maximum engine speed. Lock the stop screw in this position.
Figure 2
Information and Parts Replacement:
When requesting information concerning the pneumatic speed setting assembly, or when ordering replacement parts, be sure to include the following information:
- 1. Governor serial number as shown on nameplate.
- 2. Bulletin number (this is bulletin 04028).
- 3. The part reference number and name or description of part.
- 2. Bulletin number (this is bulletin 04028).
