1. Field of the Invention
The present invention is in the field of attenuators for stock suspensions of the type used in the manufacture of paper. It makes use of sensing means which include a sensor located upstream of the attenuator which produces a signal by means of a pressure transducer and control amplifier, which signal is fed through a high pass filter so only changes in stock pressure affect it, not the steady state pressure. Means are provided to sense the stock level in an overflow sump to produce a signal which is fed through a low pass filter and is combined with the signal from the high pass filter to operate an overflow valve so as to maintain the long term level of the overflow constant.
2. Description of the Prior Art
In the operation of a supply system for feeding a paper machine, it is necessary that the fluid suspension be delivered at a steady rate. With a paper machine stock system, pulsations in the stock suspension which could be caused by pumps or screens or the like or by disturbances such as cavitation may lead to variations in the area density of the paper in the machine direction.
It is not always possible to achieve sufficient reduction in pressure pulsations by redesign in the manufacture or installation of pumps and screens. Instead, the more usual approach is to use a properly designed and properly applied attenuator or acoustic filter which reduces the magnitude of the undesired fluctuation of the flow significantly without interfering with the steady state flow.
One type of device from the prior art which has been successfully used to minimize pressure fluctuations in a paper machine headbox is described in U.S. Pat. No. 4,030,971 which is assigned to the same assignee as the present application. The mechanism described in that patent includes a flexible diaphragm which dampens pressure variations and is supported by means of air in an air chamber divided into two compartments. Air is supplied continuously at a predetermined pressure to one of the compartments, and a relief valve is positioned in the other compartment and has a port open or closed by the movement of the diaphragm so that air is bled from the compartment as the diaphragm moves toward the stock with a drop of pressure.
It has been found in practice, however, that this type of pulsation attenuator utilizing a rubber diaphragm to separate the air and the stock is limited in its low frequency response to about one Hertz. At frequencies slightly less than this frequency of minimum effective attenuation, usually about 0.5 Hertz, the tank type attenuator can actually cause an amplification of pulsations. Consequently, it is necessary for complete pulsation control to utilize the type of pulsation attenuator described in the aforementioned patent with a supplemental device for effectively attenuating at very low frequencies.