Chemical Engineering March 2022 - 38

greater than six years should continue
to be repaired as usual and
pumps providing MTBF performance
less than two years should be scrutinized
for improvement opportunities.
Pumps with MTBF between two and
six years should be reviewed on a
case-by-case basis.
2. What is (are) the root cause(s) of
the most recent failure? The pump
owners must understand why their
pump is failing before an upgrade
or replacement pump can be evaluated.
The most effective modifications
are those that address the root
cause of past failures.
3. How is the pump performing and
how close to BEP is it operating?
4. What is the cost to restore the
pump back to the manufacturer's
specifications?
5. What is the value of process
downtime related to an unplanned
pump failure?
6. What is the cost of an in-kind replacement
pump and its delivery time.
7. What is the cost to keep a replacement
pump in stores?
8. What is the cost of design improvements
being considered?
9. What is the total cost of a centrifugal
pump hydraulic rerate being
considered?
10. What is the total cost to replace
an existing pump with one that has a
better hydraulic fit?
Repair or replace?
A common question that comes up
when pumps fail: Which pump warrants
repair and which should simply
be replaced? As described in Ref. 1,
the economic test for deciding if a
pump should be repaired or replaced
can be stated as follows: If the value
added by the repair is greater than
the repair cost minus its salvage
value, then it makes more sense to
repair the pump than replace it. This
can be expressed by Equation (1):
(1)
Where:
ms = market salvage value
mpr = market value of the pump after
repair
rva = value added to the pump by the
repair
rc= repair cost
From this relationship, we can conclude
that only repairs that produce
an added value greater than the cost
of repair makes economic sense.
This statement assumes that the
pump in question can be removed
from service and repaired without an
economic penalty.
Here are a few examples illustrating
how the value-added relationship
works:
Example 1. Consider repairing a
150-hp centrifugal pump with the
following costs:
Pump salvage value = $2,000
Post repair pump value= $20,000
Value added = $20,000 - $2,000 =
$18,000
Repair cost = $8,000
Plugging these values into Equation
(1), we see that $18,000 > $8,000.
Therefore, a repair is justified in this
case since the value
added by the repair is
greater than the repair
cost.
Example 2. Consider
repairing a 5-hp centrifugal
pump with the
following costs:
Pump salvage value =
$500
FIGURE 2. A mechanic inspects a centrifugal pump impeller during a
repair
38
Post repair pump
value = $2,000
Value added = $2,000
- $500 = $1,500
Repair cost = $4,000
Plugging these values
into Equation (1)
we see that $1,500 < $4,000. In this
case, a repair is not justified since the
value added by the repair is less than
the repair cost.
The reader will note that the lower
the replacement cost, the more likely
it will make sense to replace a pump
than to repair it. Some facilities set
a horsepower limit to define when
failed pumps are simply replaced,
which simplifies the decision-making
process for a repair shop.
Repair/spare part philosophies
Once a pump is installed, site management
decides its criticality and if
it makes sense to repair it or replace
it when it fails. Pumps that are fully
spared are considered less critical,
while pumps that are unspared (nonspared)
are considered more critical.
There are several cases to consider,
as follows:
* For smaller, less critical pumps
(<5 hp), which are uneconomical
to repair, either keep complete
pump replacements in stores or
order replacement pumps from the
manufacturer or distributor when required.
The decision to either stock
a complete spare pump or order it
when required will depend on the
expected delivery time.
* For a fully spared pump that can
be removed and repaired because
the other installed spare is deemed
to be reliable and capable of supplying
the required flow on its own: In
services with medium- to large-size
pumps, it makes sense to pull the
failed pump and repair it - either in
the shop or off site.
* For a fully spared pump that
can be removed without impacting
the process but takes too long to
repair: This might apply to troublesome
pumps that fail frequently,
which may mean that time to repair
may approach or exceed the time
between failures. In these services,
it makes sense to keep a rebuilt or
new pump in stores. When needed,
the replacement pump can be
removed from the warehouse and
installed in the field.
* For when an unspared pump's
downtime must be minimized due
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
MARCH 2022
http://WWW.CHEMENGONLINE.COM
Chemical Engineering March 2022

Table of Contents for the Digital Edition of Chemical Engineering March 2022
