Brooks Instrument SLA5800 Series - Manual (Page 26)

2-4
Brooks ® Digital MFC's & MFM's
Section 2 Installation Installation and Operation Manual
X-TMF-SLA5800-MFC-eng
Part Number: 541B027AAG
April, 2013
e. When installing a mass flow controller or meter with full scale flow rates
of 10 lpm or greater, be aware that sharp, abrupt angles in the system
piping directly upstream of the controller may cause a small shift in
accuracy. If possible, have at least ten pipe diameters of straight tubing
upstream of the mass flow controller or meter. This is not required for
meters with an integrated filter.
Special considerations to be taken when installing the SLA5853 MFC:
The Model SLA5853 has a valve design that is different from the standard
low flow Brooks TMFC's. The SLA5853 consists of a dual stage, pilot
operated valve. The pilot valve (located on the top of the MFC) controls a
differential pressure across the main valve which, in turn controls the flow
through the device. The main valve is a pressure operated valve that
utilizes a bellows spring and diaphragm to control flow. This bellows and
diaphragm assembly can be susceptible to damage by pressure spikes or
surges. For this reason, it is recommended that process line startups are
handled with care.
The bellows spring is offered in two levels. A low force for low differential
pressures (Delta P < 30psig), and a high force (delta P >30 and <300 psig).
The selection of the bellows spring is mainly determined by the differential
pressure as specified on the customer order. This should reflect your
actual process conditions.The low force bellows consists of a softer
bellows spring which is required to allow flow control at lower differential
pressures.
During startup conditions, when a process line is being pressurized, the
pressure and/or pressure differentials that the SLA5853 is exposed to may
be different from the final process conditions. For higher pressure
applications, and especially those with the low force bellows, it is important
to bring the pressure up in a controlled manner in order to prevent a
possible pressure spike to the bellows spring and main valve diaphragm. A
pressure spike could deform the bellows, damage the diaphragm or blow
out the bellows O-ring seal. This typically results in a failure to shutoff
(leakby at zero setpoint).
One method to assure successful startups is to set a 100% setpoint
command or valve override open command and then gently ramp the
pressure up to operating conditions. This will allow you to bring your
process pressures up to normal process conditions and the SLA5853
will then function as specified.
Another method is to utilize a bypass valve to allow pressure around the
device while ramping up pressure to proper operating conditions.
The main point is to not instantly open a ball valve and allow a high
upstream pressure or high back pressure surge into the SLA5853 main
valve.