Page 4 - Dixon Bayco Dry Bulk 2013_wm

800.355.1991 2 Dixon Bayco 2013
Products
Dry Bulk
Dry Bulk Products
Air Pressure Relief Valves
Theory of Operation
All Bayco air relief valves are spring-loaded system-pressure actuated devices consisting of a valve disc held in a closed
position against a valve seat by means of a spring. The pressure in the system to be protected acts on the valve disc and tends
to open the valve; however, the spring load is set to ensure that at normal operating pressures, the pressure in the system is
insufficient to open the valve. However, when the system pressure builds to a level where the pressure load on the valve disc is
equal or near to the load exerted by the spring, the valve will begin to open. If the pressure in the system were to be held at this
level, the load due to system pressure and the spring load would remain in equilibrium and the valve would be neither opened
nor closed. In such circumstances the valve will tend to flutter on the valve seat and may release a small amount of air but will
not be relieving significant pressure from the system. This point is known as the warning pressure, cracking pressure or hissing
pressure.
If the pressure in the system continues to rise, the load acting on the face of the valve will also rise and will begin to exceed the
load exerted by the spring. When the load due to system pressure exceeds the spring load, the valve will open and will remain
open as long as the system pressure remains sufficient. This point is known as the opening pressure or set pressure (also
referred to as rated or popping pressure). The difference between the crack pressure and opening pressure varies between
valves and is related to the system flow rate. However, the two should not be confused as there is a significant difference in
pressure between the two points.
If the system pressure continues to rise, the valve will continue to open and will relieve more and more air until the valve is fully
open. At this point the valve will be relieving close to its maximum airflow rate; further increase in system pressure will show only
relatively minor increases in flow rate. If the system pressure decreases, the relieving airflow rate will reduce and the valve will
start to close. However, the valve will not fully re-seat until the pressure is below the opening pressure. This pressure is known
as the Reseating Pressure and the difference between the two pressures is known as "blowdown".
In practice the valve should be matched to the system to be protected such that the maximum airflow rate of the valve is never
utilized, i.e. the valve should be capable of relieving a sufficient volume flow rate of air at the opening pressure to ensure that
the system pressure drops significantly. If the valve is open and the system pressure continues to rise above the opening
pressure then the valve is relieving less air than is being put into the system. This is a potentially dangerous situation that may
lead to over pressurization. The relieving airflow rate of an air relief valve at the maximum allowable system pressure, and
ideally at the opening pressure, should be well in excess of the system input flow rate at that same pressure.
Periodic Inspection, Cleaning and Maintenance
Dixon Bayco air relief valves are designed and built to provide accurate and consistent operation; however periodic inspection,
cleaning, and maintenance of these valves is required to ensure continued trouble free service.
Dixon Bayco air relief valves consist of a spring-loaded poppet valve that seals against a rigid annular valve seat. When the
system pressure reaches the valve opening pressure, the poppet lifts and system air passes under the poppet and out to
atmosphere. When a sufficient volume of air has been relieved, the system pressure will drop and the poppet will re-seat.
When these valves are used in applications that subject them to particulate laden air, there is potential for contamination of
the valve. Larger particles such as grains, plastic pellets etc. can get trapped between the valve seating surfaces. Applications
involving fine particles such as cement, fly ash, lime, flour etc. can result in a cementing of hardened product on the valve seat
or poppet surface. These situations can prevent the valve from fully re-seating, prevent it from opening and can lead to early
opening or leakage from the valve.
To prevent this from occurring we advise that air/vacuum relief valves be regularly inspected, cleaned and maintained. At a
minimum the valve seat and poppet sealing surface should be inspected and cleaned, and the small air vent hole in the top
casting checked for blockage. The inspection, cleaning, and maintenance should be carried out with the valve removed from the
system, and in a clean environment.
All relief valves are available with threaded connections. In addition 2180, A2180 and A2182 series air relief valves are available
with cam and groove adapter or grooved pipe connections. These types of connections make removal and installation of the
valves much easier to allow for inspections, cleaning and maintenance.
Before servicing or removing any component from the pneumatic system, the blower must be turned off and all
residual air released from the system.