Pigging Out: The Math Behind Blowing Out a Manure Line (AKA What Size Compressor do I Need?)

 When the last gallon of manure has left the pit, or it is getting close to time to switch fields, there’s still a little problem left behind: the line, filled with manure. Whether you’ve got a half-mile of dragline or three miles of mainline, it’s still full of manure. And if you just crack it open and walk away, you’ll quickly find out what a few thousand gallons of “leftover” looks like, and you won’t keep the gallons, and the cash register, flowing.

That’s why applicators finish by “pigging” the line. Pigging involves inserting a device, a foam ball (or bullet) in a launcher upstream of the pipeline and using air pressure to push it (and the manure ahead of it) to the outlet. It’s an efficient way to clear the hose. But behind that satisfying swoosh is a bit of serious fluid dynamics. So, let’s talk about the math behind pigging out your manure line.

The what size compressor do I need was also one of the common questions I saw on the Facebook page, “Manure Kings,” for a while, so I thought it would be fun to see what the math had to say.

How Much Manure Are We Dealing With?

Let’s start small, a 6-inch hose about a mile long. That line alone holds nearly 7,800 gallons of manure. If you move up to a 10-inch mainline stretching three miles, you’re talking about 65,000 gallons of material still sitting there after pumping stops. That’s a lot of fertilizer left to apply and properly place, so time to blow it out. But how much air and pressure does it take?

Doing the Math

There are two questions to answer: how much pressure do I need, and how much air do I need?

When you launch a pig and put air behind it, the pressure you need depends on three things:

·         Friction along the hose wall, and

·         How tight that foam ball fits (the pig seal friction)

·         The elevation change you need to push the manure

To estimate how much pressure you need, you can use the Darcy–Weisbach equation:

                ​

where:

𝑓 = friction factor (~0.02 for turbulent flow),

𝐿 = hose length in feet

𝐷 = hose diameter in feet

𝜌 = manure density ~63 lb/ft³

𝑣 = velocity of the manure that you want to maintain in ft/s (I’m going to aim for 5 ft/s)

Referring back to our examples, I calculate for the 6-inch line, I’d need 36 psi plus whatever the elevation head change requires, and for the 10-inch line, I’d need 65 psi plus whatever the elevation head change requires. If I have the pig moving at 5 ft/s it takes a little under 20 minutes to blow that mile of 6-inch line, but almost an hour to blow the 10-inch line.

Now, we’ll need a little extra pressure to overcome the pig’s drag, but I don’t have a great handle on estimating that. The good news is, as long as we have enough pressure, the manure will move—it’s just a question of how fast.

But how much compressor flow do I need?

Here’s where the flow rate comes in. The compressor doesn’t just need to hit that pressure; it must keep supplying air fast enough to maintain the pressure as the pig advances. If the compressor can’t keep up, the pressure drops, and the pig slows or stops if we end up with no pressure. Sure, it will start again, but we want to have a guess of how long it might take to blow out the line.

The air behind the pig is filling a growing volume of hose as the pig moves forward. The faster the pig moves, the faster the air volume expands, and the faster your compressor has to supply air.

If your 6-inch pig is moving at 5 feet per second, that’s about 60 CFM of hose volume per minute being created behind it. For the 10-inch line, that’s about 163 CFM of hose volume.

Let’s check this against a real-world example: Say I have a compressor with a rated capacity (not an endorsement, but one that had a rated value for me to use) of 185 CFM at 100 PSI. For the 10-inch line, this sounds pretty close, as I said I wanted 163 CFM, but I estimated I only needed 65 psi plus the drag of the pig and elevation change, so let’s call it 80 psi. Given those characteristics, the compressor should work and move everything a little faster than I estimated.

In other words:

·         Pressure gets the pig moving.

·         Flow rate controls how fast it moves.

·         Both determine how long the job takes

Practical Tips Before You Blow

Pigging with air can be deceptively dangerous. You’re storing a lot of energy behind that foam ball.

·         Segment long lines. Don’t try to clear three miles in one go; break it into manageable lengths with valves or disconnect points.

·         Start slow. Ramp up the air gradually while watching a pressure gauge.

·         Be safe, high pressures and air’s want to decompress rapidly make this one of the most dangerous parts of the job.