Hydrocarbon Processing - January 2021 - 66

Process Controls, Instrumentation and Automation
updates each sensor reading
individually. This covers all four
temperature measurements.

FIG. 2. An acoustic monitor mounted on
the valve outlet can detect a full opening
or slow leakage.

* The DP reading across the
tubes uses a native WirelessHART
DP transmitter.
* The flow meter for the transfer
fluid can use a self-contained DP
transmitter that is available with a
native WirelessHART network.
When the temperature change and
flowrate of both fluids are known, it is
possible to determine how close the heat
exchanger is running to its theoretical
limit. If efficiency is low due to fouling,
then information from the instruments
can help determine which side of the
tube wall is experiencing deposit formation. If there is a rise in the DP reading
across the tubes unrelated to a change in
flow, this suggests that the deposits are on
the process side. A DP reading across the
shell side is not recommended, since the
internal free-passage flow is much larger.
If there is enough depositing to cause a
significant change on the shell side, this
could cause a drastic loss of efficiency.

FIG. 3. Wall thickness sensors, combined with monitoring software, can provide data on realtime conditions and metal loss rates.

FIG. 4. Output from multiple sensors can be synchronized in time, using specialized software
to provide a heat map of metal loss.

66 JANUARY 2021 | HydrocarbonProcessing.com

Pressure relief valves. Pressure relief
valves (PRVs) have no built-in mechanism capable of reporting their condition
or activity, so operators have little indication of their status. A PRV retains its seal
by having a spring hold the stem against
the seat. A fully closed valve is quiet because nothing is flowing through it. When
the system pressure exceeds the setpoint,
it opens, releasing the contents of the system-either liquid or gas (or both). This
creates turbulence, generating noise that
an acoustic monitor (FIG. 2) can hear and
report to the automation system.
When pressure recedes, the valve
should close again, and the noise should
cease. Data from the acoustic monitor
can report the time that the discharge
began and ended, while indicating how
serious the discharge was, based on
sound amplitude.
The problem is getting the valve to reseal. Since overpressure incidents are often
related to process upsets, particulates can be
stirred up and blown out with the contents.
Some can lodge in the valve seat, leaving it
in a perpetually leaking state, which operators call " simmering. " An acoustic monitor
hears the simmering, so operators can tell
if a given PRV has fully reseated itself after
an incident. Maintenance can then decide
when to address the issue.
With continuous monitoring, technicians can tell if a pressure release is underway or if a valve has not fully resealed itself.
With prompt action, the overall effectiveness of the unit's safety system can be assured, while avoiding product loss and potential environmental consequences.
Steam traps. Steam traps are deployed
throughout steam distribution systems to
remove condensate. They are subject to
mechanical problems, either failing open
(resulting in a steam leak) or failing closed
(causing condensate slugs to back up into
steam lines).
Most plants monitor steam traps on
manual rounds, where maintenance technicians look for signs of leaking, either
visually or with handheld acoustic monitors. Condensate slugs are usually discovered when they damage equipment. Estimates suggest that 18% of the steam traps
in a typical large refinery or petrochemical
plant fail every year, each causing about
$16,000 in extra fuel and steam costs.
By installing WirelessHART acoustic
transmitters, plants can quickly identify

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Hydrocarbon Processing - January 2021

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