Some liquids are not easy to pump, and ammonia is probably one of them. A number of years ago, a reader contacted us regarding issues he encountered with pumps in ammonia service. A bit of exasperation caused him to write to two colleagues with whom he shared concerns and questions relating to pump performance. We quote him here:
“Yes, I am still collecting articles on NPSH problems at low flow on centrifugal pumps. The most recent is in the November 2013 issue of a particular trade journal which I receive free of charge. It states that 29 years of Texas A&M International Pump User Symposium Proceedings, [never heard of them–have you?], deal with the problem of increasing NPSH requirements at low flows. The article also mentioned that Igor Karassik covers the same thing in his old pump texts, [never heard of him either]. Despite these revelations, I have yet to get anyone’s attention at an industry association representing parties interested in Ammonia Refrigeration. For a certainty, various ammonia pump manufacturers continue to publish pump curves that do not show NPSH at low flows. Problems with NPSH at low flow caused a disastrous parade of bearing failures at a large facility that I was drawn into as a troubleshooter….and I have not heard if any solution has been applied to correct the problem.”
Disappointed by not getting others to reply, this individual wrote to us, as follows, and referred to the 2013 article:
“I find your comments on NPSH at low flows very interesting and fully agree with your conclusions. This is reflected in my e-mail [above] to a couple of peers. I have copied you to get an idea of where we stand on this problem in the refrigeration industry. As you well know, in our industry the problem of NPSH is exacerbated because we are always working with a volatile refrigerant, whether ammonia or something else. Do the pump supplier curves you work with have the same lack of NPSH at low flows? The suppliers we work with not only aren’t showing NPSH at low flows but do not even state a warning about operation at low flow. I am trying to light up our industry association to do some investigation and research on this, but would appreciate your response.”
We replied by stating that knowledgeable manufacturers of ammonia pumps have access to successful user sites. Therefore, knowledgeable suppliers of large-scale refrigeration equipment should have an existing facility’s NPSH-available (NPSHa) readily available. But why is his client operating ammonia pumps at low flow in the first place? Would a decent replacement impeller operate more optimally? If the pump manufacturer does not offer the right impeller, a non-OEM would be pleased to design and manufacture what the client needs (Ref. 1).
Let’s return to the story. As long as the pump inlet friction losses and pump-internal heat profiles are known, an NPSHa of 2 or 3 ft over the (true) NPSHr should be sufficient. The pump manufacturer should have knowledge of the true NPSHr, taking into account suction losses (reduced pressure right behind the suction nozzle) and temperature-pressure behavior inside the pump (Ref. 2). Pump manufacturers may (or may not) publish NPSHr at points away from the best efficiency point (BEP). They sometimes reason that publishing these values could be misunderstood; they see it as an invitation to operate at less than BEP throughput, which causes internal recirculation (a possible source of vaporization) and bearing load-related problems. Other vendors believe that publishing NPSHr values commensurate with lower than BEP flows will invite the various pump competitors to make claims and counterclaims (a “bidding war,” so-to-speak).
AS TO PRECEEDINGS OF TAMU PUMP SYMPOSIA
Regarding Texas A&M’s International Pump Users (“TAMU Pump”) Symposia, these events have been presented for the past 30 years (usually in Houston). The symposium is co-located with the Turbomachinery Symposium (“TAMU Turbo”). The two conferences draw about 6000 attendees and close to 400 exhibitors. Both TAMU Pump and TAMU Turbo (since 1971) represent fine networking opportunities.
HONORING A WISE PUMP ENGINEER & WISE BUYERS
Igor Karassik (1911-1995) was one of the world’s foremost pump experts (Worthington’s Chief Engineer) and, after his retirement in the mid-1970s, the first recipient of the Worthington Medal. He served on the advisory board in the early six or so years of TAMU Pump symposia; he was also a prolific writer and superb communicator with many books and over 1,000 articles to his credit. The Internet contains many entries on him.
THE BOTTOM LINE
Finally, keep in mind that wise buyers have a very simple procurement strategy. Those at Best-in-Class (“BiC”) companies in the HPI (Hydrocarbon Processing Industry) never purchase critically important machines without thoroughly checking user experience.
As a rule, wise buyers do three important things that their not-quite-as-good competitors have either forgotten or think they have no time to read about:
1. They have a bidders’ list. Unqualified bidders do not receive an invitation to bid.
2. Their cost estimating manuals show the cost of reliable equipment, never the cost
of cheapest equipment. Best-in-class companies want reliability, not lowest initial cost.
3. They allocate an additional 5% of the equipment cost to do up-front Machinery
Quality Assessments (“MQA”).
These three are key action items are common to all BiCs. Capturing the accolades these companies enjoy requires effort and the effort is worth every penny of investment. As to engineers and why they are not always respected, it’s fair to point out that some tend to offer opinions instead of facts. Getting at facts may take time (including time away from the office). Reading, studying, and communicating facts are not merely fruitful technical endeavors: Engineers who consistently engage in these activities will also be respected.TRR
1. Perez, Robert X., and Bloch, H. P., Pump Wisdom: Problem Solving for Operators and Specialists, 2nd Edition (2022), John Wiley & Sons, Hoboken, NJ.
2. Bloch, H. P., and Budris, A. R., Pump User’s Handbook: Life Extension, 4th Edition (2013), The Fairmont Press, Lilburn, GA.
ABOUT THE AUTHOR
Heinz Bloch’s long professional career included assignments as Exxon Chemical’s Regional Machinery Specialist for the United States. A recognized subject-matter-expert on plant equipment and failure avoidance, he is the author of numerous books and articles, and continues to present at technical conferences around the world. Bloch holds B.S. and M.S. degrees in Mechanical Engineering and is an ASME Life Fellow. These days, he’s based near Houston, TX.
Tags: reliability, availability, maintenance, RAM, ammonia pumps, TAMU Pump Symposium, TAMU Turbo Symposium