BEYOND BACK-TO-BASICS: PROCESS PRINCIPLES AND CONCEPTS-3: Maintaining seal-flush pressure in centrifugal pumps

"A typical suction-piping velocity might be 3 fps before the liquid flows into the eye of the pump's impeller and accelerates, let's say, to 6 fps. That flow acceleration to 6 fps from 3 fps requires energy."

"Does that acceleration energy come from the pump's impeller?"

"No, Donald, it comes from the potential energy of the liquid, or from the head of liquid."

"Is that what engineers call the required net positive suction head (NPSH) of the pump?"

"It's exactly that," I said.

"And if the required NPSH isn't smaller than the head of liquid available in the vessel, then I need to raise the vessel liquid level, right? Otherwise, my pump's going to cavitate?"

"You got it, Donald," I said. "You'll tear up the pump's mechanical seal due to vibration."

"This is interesting, Mr. Norm. So what happens to the liquid after it flows into the eye of the impeller?"

"The liquid spins around inside the impeller, where it accelerates from 6 fps to 100 fps as it discharges from the impeller's vane tip (Fig. 1)."

"But where does that energy to accelerate the liquid come from? Also from the available NPSH?"

"No, it comes from the electric motor, or the steam turbine, spinning the impeller."

Donald paused for a moment to consider this, and sensing his confusion, I explained the process in more detail.

According to Bernoulli, electricity-a form of energy-can be converted to kinetic energy, which is what's happening inside the pump's spinning impeller.

The pressure of the liquid discharged from the impeller is slightly lower than the pump suction pressure. The impeller discharge pressure is the pressure inside the entire pump case. To ensure the pump's seal doesn't experience pressure much above the pump suction pressure, there are pressure-balancing holes drilled through the impeller, which I pointed out to Donald.

"Yeah, I've seen those holes before. Jerry told me they were there to equalize pressure on both sides of the impeller. Is that right?"

"Yes, Donald, Jerry is correct. But that pressure he's referring to is the pump-suction pressure, not the discharge."

"Mr. Norm, I'm still confused. None of this makes any sense. How does the pump-discharge pressure increase from the 10 psig suction to the 100-psig discharge?"

Converting velocity to head

To explain the concept, I had Donald look at the discharge nozzle of an unflanged pump.

"Do you see how the discharge flange is 6 in., but the opening down inside the pump case is a lot smaller?"

"Yes, sir, it looks like it's about a 1.5-in. opening."

"It follows then (from Fig.1), that the velocity at the 6-in. discharge flange has dropped from 100 fps down to 6 fps (100 fps x [1.5 ÷ 6]2 = 6 fps). The reduction in velocity has been converted into…"

"Hey, I think I get it, Mr. Norm!" Donald interjected with excitement.

"Whoa, Donald, that's great. Just to make sure, though, go ahead and explain it to me."

"It's just like a roller coaster car slowing down as it goes uphill. The speed of the car is converted to elevation, like what you call feet of head or what that Swiss guy Bernoulli calls potential energy. But that doesn't happen until the liquid's out of the pump itself, which is why the seal-flush pressure only needs to be a little bit higher than the pump's suction pressure."

"Okay," I said. "Keep going."

"The discharge pressure really doesn't affect the pressure that the pump's mechanical seal has to hold, but even if the seal-flush pressure did build up a bit behind the impeller, that pressure would bleed itself back to the pump suction through those pressure-balancing holes drilled in the back side of the impeller. Yeah, I got it!"

And he did, for the most part. But Donald's questions didn't end here.

Head, discharge pressure

"Mr. Norm, you keep talking about head, but that doesn't mean anything to me. As an operator, I only see pressure. Doesn't the centrifugal pump create a certain amount of discharge pressure?"

"Well, yes and no, Donald."

"Could you make up your mind? Does a centrifugal pump increase the pressure of the liquid it's pumping?"

"A centrifugal pump, which is pumping a nonviscous product, develops a certain amount of feet of head at a given volume of flow, regardless of the density or specific gravity of the liquid pumped," I said. "To be more clear..."

"Mr. Norm, you couldn't be less clear," Donald said. "But go ahead. Talk slowly and I'll listen."

"As I was saying, Donald, since we know that water has a specific gravity of 1.0, and that 1.0 psi of pressure equals 2.31 ft of water, we can use the following equation to convert from feet of head-which is what the pump creates-to pressure:" DP = (DH × sp gr) ÷ 2.31, where DP = the increase in discharge pressure over suction pressure in psi, DH = increase in feet of head developed by the pump in ft, and sp gr = specific gravity (density) of liquid at flowing temperature relative to the density of cold water at 60° F.

"So a pump that's pumping diesel will create more discharge pressure than one pumping naphtha because diesel is denser than naphtha?"

"Correct, Donald."

"Okay, I get the specific gravity idea," Donald said. "But how about the nonviscous stuff? What's all that mean?"

"Well, 90%+ of the liquid we handle in a refinery is low-viscosity material, meaning its less than 50 cst. In our plant, the only high-viscosity stuff is cold Venezuelan crude, or cool slop wax from the vacuum tower. We'd handle anything that has a viscosity of more than 50-100 cst with a positive displacement pump, not a centrifugal pump."

"Let's just stick with the centrifugal pump for now," Donald said. "You mentioned earlier about feet of head at a given volume of flow, and I'm not sure I get what you mean. Could you walk me through it?"

Pump curve

"A centrifugal pump operates on a performance curve (Fig. 2). The head depends on the volume of liquid. So whether you're pumping water with 1.00 sp gr or gasoline with 0.75 sp gr, a pump will produce the same feet of head as long as the flow is constant."

"Okay," Donald said. "But how about that horsepower curve? I guess that's the same for water and gasoline also?"

"I know it's confusing, but no," I explained. "The motor horsepower-or the amount of amps the motor will pull-is a linear function of the specific gravity. If the specific gravity increases by 20%, so will the motor amps. Manufacturers develop pump curves using water in a test rig. Even though a pump may be designed for diesel with a 0.85 sp gr, the horsepower shown on the right-hand vertical axis of the pump curve is likely the required horsepower for water."

"Man, that's really confusing," Donald said. "Even so, Mr. Norm, I'm glad you clarified it for me because it explains why P-403 trips off on high-motor amps when I switch from water to caustic, unless I throttle back on its discharge first."

"Right, Donald, the specific gravity of the water is 1.00, while the caustic has a 1.20 sp gr. At the larger specific gravity, you're going to be limited by motor horsepower, not the pump's capacity."

Positive displacement pumps

"Mr. Norm, it's near the end of my shift, but before I head to the gate, I have one more question. Out in the tank field, we have a crude pump that I've been told is different from all the centrifugal pumps on our unit. It looks the same as our centrifugal pumps. What's the difference?"

"I get confused by this also, Donald. While they look the same, that pump in the tank field is a positive displacement pump."

"You mentioned that type of pump earlier. So what's the difference?"

"An important one," I said. "As the performance curve shows, centrifugal pumps develop the same feet of head for a particular flow rate. Positive displacement pumps, however, don't run on a curve. Instead, they generate as much head or discharge pressure as they need to overcome the pressure of the system into which they're pumping."

"Translation, Mr. Norm?"

"Think of it this way, Donald. If the pump needs to push 100,000 b/d of crude into a tank, its discharge pressure might be just 25 psig. If the pump needs to push 100,000 b/d of crude into your crude unit, its discharge pressure could be 150 psig."

Seal-oil flow to idle pumps

What began as my answer to Donald's question had evolved into a discussion about the fundamentals of centrifugal pump operations.

"Wow, Mr. Norm, that's a lot to take in, but it's the best explanation I've ever gotten, including from books, on how pumps actually work. I'm going to reset the seal-flush pressure just a bit above the pump's suction pressure on all my pumps tomorrow, just like you told me. By the way, does that go for the spare pumps we aren't running?"

"No, and I'm glad you asked," I said. "But now I have a question for you. Do you know why the answer's no?"

After a brief silence and with a slightly panicked look in his eyes, Donald shook his head from side to side.

"Donald," I said in a gruff voice. "I'm teasing you."

"Wow, Mr. Norm, you really had me going there for a second," Donald said, smiling now in relief.

"Actually, I'm only partly kidding because the answer is important," I said.

"Alright, Mr. Norm, why is the answer no?"

"Take your resid pumps, which use diesel for seal flush," I said. "When you shut the pump down, you wait a few minutes for the diesel oil to flush through the pump and then block the diesel seal flush in. Even though the pump isn't running, the seal-flush flow will still be around 60% of its flow when the pump was in operation. Every barrel of diesel you put into resid is costing the refinery about $10. Let's not waste seal flush on idled pumps."

"Man, they should have told me that when I was hired," Donald said. "I've been leaving the seal flush lined up to my idle pumps for years. Someone should've told us operators before not to do that. What a waste!"

"You're right, Donald, a big waste," I said. "But the important thing is that you know now. So walk in the light while there is light, my friend."

"Will do, Mr. Norm."

General Introduction

Geared to young and seasoned professionals alike, "Beyond back-to-basics: Process principles and concepts" is a new series of articles designed to present a straightforward approach to mastering the principles and concepts all process engineers should be able to apply without the need of a computer.

While simulations and models are useful for examining long-term operational issues, they cannot replace the dimension of human logic and reason required when tackling the array of complex-and sometimes life-threatening-situations that occur in process plants.

Using experiences from the author's more than 50-year career in the process industry, articles in the series will provide approaches to understanding core process concepts in ways that will equip the engineer to walk out of an office, into a plant, and directly resolve process deficiencies via small operational changes or simple retrofits.

This is the third article in the series.

Safety note

Autoignition is the temperature at which hydrocarbons can burn without a source of ignition. Except for hydrogen, the heavier a hydrocarbon is, the lower the temperature it requires to autoignite.

Gasoline has an autoignition temperature of 480° F. Asphalt, when spread over a porous surface like clothes, autoignites at 300° F. This is why a seal leak on heavy vacuum gas oil pumps is so dangerous.

It's much safer to get a sample of naphtha at 350° F. than vacuum resid at the same temperature.