Piping is the least glamorous part of any vacuum system. Nobody gets excited about pipe runs, elbows and valve positions. And yet a surprising amount of poor performance traces back to exactly this. A cooler that’s fine on paper but sits in the wrong spot, a trapped air pocket nobody vented, a seal liquid line that starves the pump on startup, a strainer buried where no one can reach it. If you run nash liquid ring vacuum pumps, the loop around them matters nearly as much as the pump itself, because seal liquid has to arrive at the right temperature, the right flow and without air. Getting the layout right is mostly common sense plus a handful of rules people forget. It’s also an area where good heat exchanger suppliers can save you real trouble, if you let them see the drawings early. So let’s walk through it.

Piping decisions nobody revisits

Here’s how it usually goes. A pump gets selected, a skid or a footprint gets drawn, and somebody sketches the pipework to connect things. It works well enough on commissioning day, so nobody touches it again. Years later, the same layout is still there, with all its quirks, and everybody assumes it’s fine because it always was.

But layouts have consequences. A low point in the wrong place collects sludge. A long dead leg grows slime. A high point without a vent traps air that silently blocks heat transfer. A cooler mounted where you can’t pull its covers off never gets cleaned. A sensor placed too close to a bend gives readings nobody trusts.

These things don’t announce themselves. They show up as strange symptoms. Seal temperature that wanders for no obvious reason. A pump that’s noisy until it warms up. A cooler that performs worse than its datasheet says it should. Operators learn to live with it, and the quirks become part of the plant’s folklore.

The good news is that piping is cheap to fix compared with replacing equipment, especially at the design stage. A few sensible rules, applied up front, prevent most of these headaches. The rest of this post is basically those rules, explained the way I’d explain them standing next to the skid.

The seal liquid loop, from separator to pump

Start with the heart of the system. In a recirculating arrangement, the separator collects the mixture leaving the pump, lets gas escape, and holds a volume of liquid. From there, liquid flows to the cooler and back into the pump.

The first rule is making sure the pump never starves. Seal liquid has to reach the pump inlet reliably, at the right flow, even at startup. If the separator sits above the pump, gravity helps, and you want pipework that doesn’t trap air or create siphons. If a circulation pump is needed, it must have enough suction head, called net positive suction head, to avoid cavitating. That usually means keeping suction pipes short, large in diameter, with few bends, and the separator liquid level well above the circulation pump inlet.

Size the pipes sensibly. Too small and pressure drop starves the flow. Too large and velocities drop, letting solids settle. Follow the pump maker’s recommended seal liquid flow range, and ask the cooler supplier for pressure drop at that flow, then add up the whole loop’s resistance.

Avoid air entrainment. Air bubbles in the seal liquid reduce effective capacity and cause noise. Keep return lines submerged where they enter the separator, avoid free falls and splashing, and make sure vents and level controls work.

Think about start up and shutdown. When the pump stops, liquid should stay where you want it, not drain backward out of the pump or siphon through a cooler. A check valve or a loop seal, correctly placed, can help.

Where the cooler sits and why

Cooler position in the loop deserves thought. Two common choices exist. The cooler can sit in the main seal liquid line between the separator and the pump, cooling all of the recirculated liquid. Or it can cool a portion, in a side stream, with the rest bypassing.

In the main line arrangement, the cooler sees the full flow. That’s simple and gives tight temperature control, but it adds all of the cooler’s pressure drop to the loop, and the cooler must pass the full seal liquid flow without clogging.

In a side stream, only part of the flow goes through, which can allow a smaller cooler and lower pressure drop, though temperature control is less direct.

Where it physically sits matters too. Putting it below the separator liquid level means it stays flooded, which helps avoid air pockets and keeps the suction side full. Putting it above creates a high point that traps air unless it’s vented. Often the best spot is on the pump discharge side of the circulation pump, where pressure is higher and air is less likely to cause trouble, but check your design.

Leave room. You need space to pull covers or a tube bundle, to open plates, or to connect cleaning hoses. Place the cooler where you can reach it, with clear access and lifting points. A cooler nobody can get at is a cooler that never gets cleaned.

Keep it close to the pump if possible. Long runs add resistance, heat loss and cost, but don’t squeeze it so tightly that maintenance becomes impossible.

Venting, draining and trapped air

If I had to pick the most common piping fault, it would be trapped air. It’s everywhere, and it’s easy to prevent.

Air collects at high points. In a cooler, an air pocket covers part of the heat transfer surface, cutting performance without any obvious sign. In pipework, it can cause noise, flow instability and even gas locking in a circulation pump. After maintenance or refilling, air gets introduced, and unless there’s somewhere for it to escape, it stays.

So put vents at every high point in the seal liquid and cooling water circuits, including at the top of the cooler. Make them accessible, with a valve you can actually reach, and pipe the outlet somewhere safe. During filling, vent until liquid flows steadily. Check again after the system has run for a while, since dissolved air comes out of solution as the liquid warms.

Equally important is draining. Every low point should have a drain, so you can empty the cooler and pipework for cleaning, repair and freeze protection. Slope horizontal pipes gently toward the drain points, rather than leaving them perfectly level where liquid can pool.

Avoid dead legs. A capped stub or an unused branch holds stagnant water that grows slime, corrodes, and can freeze. If you need a branch for future use, keep it short, or put the valve right at the main line.

Think about U shaped sections that act as traps, both for air and for liquid. They make draining and venting a nightmare. Layouts that rise and fall steadily are much easier to look after.

Cooling water piping and connections

The other side of the cooler has its own rules, and connecting it correctly makes a real difference to performance.

Flow direction first. Most coolers are most efficient in counterflow, with the two fluids moving in opposite directions. So the cooling water should enter at the end where the seal liquid leaves, and leave at the end where the seal liquid enters. Connect it backwards, parallel flow, and you lose performance quietly, sometimes noticeably. Check the manual, and label the connections clearly, so nobody swaps them during maintenance.

Position the connections so the cooler stays full of water in use, and drains completely when needed. Generally, cooling water enters low and leaves high, helping to push air out, though specific designs may differ.

Size the cooling water pipes for sensible velocities. Too slow encourages sediment and slime. Too fast causes erosion and noise. Ask the cooler supplier for the pressure drop at your flow, and make sure the supply pressure can deliver it.

Control matters. If a temperature controlled valve regulates cooling water, put it on the outlet or inlet according to the control design, with enough straight pipe to ensure steady flow, and size it for the real flow range. Consider a minimum flow bypass so the cooler never runs below the velocity needed to keep it clean.

Protect against surges. Slamming valves cause pressure spikes that fatigue the cooler. Use slow acting valves where possible, and soft start arrangements on pumps.

And think about where the cooling water goes after the cooler. Discharge limits, recirculation to a tower, or reuse should all be planned.

Valves, strainers and bypass for maintenance

Maintenance access should be built into the piping, not bolted on afterwards. A handful of fittings make the difference between a thirty minute job and a day long shutdown.

Isolation valves on both sides of the cooler, on both circuits, let you shut it off and drain it without stopping everything else. Make sure they’re full bore, quality valves, since a leaking isolation valve ruins the whole point. Place them close to the cooler, with unions or flanges so it can be removed.

Strainers upstream of the cooler on each circuit catch debris before it gets inside. Choose a mesh suited to the cooler’s passage size, and install them where you can open and clean them easily. Add a differential pressure gauge across the strainer if you can, so you know when it’s blocked. A neglected strainer causes its own problems, starving flow, so it needs a place on the maintenance schedule.

Cleaning connections are worth their weight in gold. A pair of flushing ports, with valves, on each side of the cooler lets you connect a temporary cleaning loop and reverse flush without breaking pipework. It also reduces the chance of leaks from repeated flange opening.

A bypass around the cooler, with proper valves, lets the pump keep running while the cooler is cleaned or replaced, at least for a short time. It’s not always possible, depending on duty, but where it is, it buys valuable flexibility.

Add instrument tappings. Pockets for temperature sensors and tappings for pressure gauges, on both sides in and out, make performance monitoring easy. Place them in straight runs, away from bends and valves, for reliable readings.

Supports, expansion and insulation

The last group of rules deals with mechanical and thermal details that often get skipped.

Support the pipework independently. A cooler isn’t a pipe support, and connections shouldn’t carry the weight of heavy pipes, valves or strainers. Hangers and supports near the connections stop stress building up, and prevent leaks and cracks. Don’t force misaligned flanges together. Cut and fit a proper spool piece instead.

Allow for thermal movement. Hot lines expand, and if they can’t move freely, they push on the cooler and the pump. Expansion loops, flexible connectors or sliding supports absorb movement. Vibration is another consideration. Flexible connectors near the pump reduce transmitted vibration, though they need to be selected for the pressure and fluid.

Insulate where it matters. Hot pipes lose heat and pose burn risks, and cold pipes sweat, dripping condensation onto electrics or floors. Insulating seal liquid lines helps keep temperatures steady and protects people. In cold climates, insulation and heat tracing prevent freezing, particularly on cooling water lines and drains.

Choose materials carefully. Mixing metals in the presence of water can cause galvanic corrosion, so use compatible materials or insulating fittings. Check the pipework suits the water chemistry, including chlorides, and the cleaning chemicals you plan to use.

Label everything. Flow directions, valve functions, circuit names. Clear labels prevent mistakes during maintenance, especially when different people service the system.

Finally, keep drawings up to date. If you change the pipework, update the diagram. An accurate drawing is a gift to whoever has to troubleshoot at three in the morning.

What heat exchanger suppliers should check with you

All of this is much easier if you involve your cooler supplier early. Good heat exchanger suppliers do more than quote a unit. They look at how it will be installed, because the installation decides how it performs.

Show them your layout, even a rough sketch. A good supplier will ask where the separator and circulation pump sit, how the cooler connects, whether there’s room to service it, and how air will be vented. They’ll check connection orientations, flow directions, and recommended mounting positions.

They’ll talk about pressure drop across the whole loop, not just the cooler. They’ll ask what strainers and filtration exist, and recommend mesh sizes. They’ll ask about isolation and bypass arrangements, cleaning connections, and instrument points.

They’ll want to know about your cooling water supply and control, including flows, pressures, temperatures and quality. They’ll ask about freezing risk, thermal movement and vibration, and how the unit will be supported.

They should offer practical advice. Where to put vents and drains. How to connect for counterflow. What clearances to leave for bundle removal or plate access. What to avoid. And they’ll be honest if your proposed layout creates problems.

Ask them directly. What would you change in this layout? What happens if air gets trapped here? How would we clean this? Suppliers who answer thoughtfully and specifically are the ones worth keeping, and they’ll still be reachable after the equipment arrives.

Conclusion

So, how should you pipe the cooling loop around vacuum pumps? With the pump’s needs first, reliable seal liquid at the right flow and temperature, with no air and no starvation. Place the cooler where it stays flooded and can be reached. Vent every high point, drain every low point, and avoid dead legs. Connect cooling water in counterflow, size the pipes sensibly, and protect the cooler from surges. Add isolation valves, strainers, cleaning connections and instrument points, support the pipework properly, and allow for thermal movement. For nash liquid ring vacuum pumps, that discipline keeps the seal liquid steady and the vacuum stable.

And bring your supplier into the conversation early. Good heat exchanger suppliers review the layout, flag problems and give practical advice that saves trouble later. Do that and the loop around your pumps will quietly do its job for years, which is exactly what plumbing should do.

FAQs

Where should the cooler go in a recirculating seal liquid loop?

Commonly in the main line between the separator and the pump, where it cools the full flow and gives tight temperature control. It should stay flooded, be easy to reach and be vented at any high point.

Why is trapped air such a problem?

Air pockets cover heat transfer surfaces, cutting cooler performance without obvious signs, and can cause noise, unstable flow and gas locking in circulation pumps. Vents at every high point prevent most of it.

How should cooling water be connected to a cooler?

Normally in counterflow, so cooling water enters where the seal liquid leaves and leaves where it enters. Connecting it backwards reduces performance. Check the manual and label connections clearly.

What valves and fittings help maintenance?

Isolation valves on both circuits, strainers upstream of the cooler, cleaning and flushing connections, a bypass where possible, and tappings for temperature and pressure instruments in straight runs.

How do I avoid stress on cooler connections?

Support pipes independently, never let the cooler carry pipe weight, fit proper spool pieces instead of forcing flanges together, and allow for thermal movement with expansion loops or flexible connectors.

What should heat exchanger suppliers check about my layout?

They should review connection orientation, flow direction, mounting position, access for servicing, venting and draining, pressure drop across the loop, strainers and filtration, cleaning arrangements and freeze protection, and suggest improvements.