Rosemount 3491 - Page 84

Reference Manual
00809-0100-4841, Rev. AA
May 2007
Rosemount 3490 Series
4-52
PV limits Relay
This relay mode uses the On/Off points (e.g. P411 and P412) as an Alarm
relay. The points are used as limits for the PV value (D800). Limit values can
be in any order - the relay is energised while the PV value exceeds the higher
limit value or while it is below the lower limit value.
Rate of Change Relay
A rate of change value for the parameter D800 (PV value) is calculated every
5 seconds in units of PV per minute:
D809 = (PVnow - PV5 seconds ago) * 12
The On and Off points of a Rate of Change (RoC) mode relay are used as
high and low limits for D809. Limit values are in units of PV per minute
(PV/min) and can be in any order - the relay is energised while D809 exceeds
the higher limit value or falls below the lower limit value.
Typically, where the PV value is a liquid level measurement, the RoC relay
can be used to warn of a quickly rising (or falling) liquid level. Alternatively, the
RoC mode relay can be used for controlling the rate of liquid flow.
Also, see “Pump Efficiency Alarm (Models 3491 and 3492 only)“ on page 4-57
and “Pumped Volume Totalising“ on page 4-59 for further uses of D809.
Auto-Sequence (Models 3491 and 3492 only)
(Special Control Function - see Table 4-12 on page 4-45)
Optional rotation of leading (most used) relay can be applied to Assist or
Standby mode relays, but not both. To use these options, two or more relays
must have the same mode. The lowest numbered relay is initially the lead
relay.
NOTE:
Rotation of relays is performed without the actual swapping of values between
relay parameters.
P270 Auto Seq Enable (Factory default is "Off")
Select a rotation auto-sequence. All options are summarised in Table 10
(below).
P271 Auto Seq Qual (Factory default is 0)
This defines the threshold to be established (e.g. how many times, ratio of
starts, etc.) before applying an auto-sequence to rotate the 'lead' to the next
relay with the same mode.
Table 4-13.
Auto-Sequence Options
Option Rotation basis
Standby Starts For Standby mode relays only - rotation is based on how many times
the 'leading' relay has been energised compared to parameter P271.
Stdby On Time For Standby mode relays only - rotation is based on the hours that the
'leading' relay has been energised compared to parameter P271.
Stdby Ratio T For 2 Standby mode relays only - rotation is based on the ratio of ON
time for 2 relays compared to P271.
(1)
Stdby Ratio S For 2 Standby mode relays only - rotation based on the ratio of starts
(times energised) compared to P271.
(1)