Too little and too much pump

Henry

For a few years now, we have been replacing genoni boiler heat exchangers on installs by others. In all cases, there was insuficient pump. They originaly sized the pump for 40F differential. So they saved a few hundred on each pump. We use 25 or 20F for a differential across the boiler. It keeps the velocity within parameters that prevent sedimentation in the heat exchanger. We have not had one failure due to the boiler pump.

In a large building there were two Camus Dynaflame 2 million BTU each near condensing boilers (85%). The right boiler had its heat exchanger replaced early 2014. In december, the left boiler heat exchanger failed. I offered them a S/S version which is now standard because the boiler was condensing. (It was a very poor design by a well known international control company) We replace the unit with a S/S. 13 months later the new S/S exchanger failed due to a bad weld. It was under warranty so in January we replaced it. This january one year later, that one failed. It was sent to Camus and was taken to autopsy. Several tubes were blocked. The client now had to pay for a new heat exchanger. I also have proposed a bag filter. The International control company and its engineer were saying that the S/S exchanger had less flow because the tubes were smaller than the copper exchanger. A meeting was scheduled for last Friday. So, I did my homework. In fact, the S/S exchanger had 3 tubes per pass compared to 2. It had near 1 ft less restriction. Then I checked the pump curve. It was oversized. There were a balancing valve for each boiler but it was fully open. It is typical for engineered jobs to oversize the pumps but use the balancing valves to get the proper flow. The pump needed a minimum of 19 ft of head. It had around 6 ft. Therefore it was cavatating and the solids that were suspended dropped to the bottom of the heat exchanger. I had them balance the flow for 20 degF delta T. I have seen this happen several times over the years and saw another one this morning. You must maintain at least 20F diff and a max of 35 F to prevent sedimentation.

Here are the pics of the autopsy

jumper

Boilers can have different flow rate requirements from the rest of the system.But primary-secondary waste energy.

deadmansghost

Any figures about the percentage of makeup on that system?

Henry

A water meter was installed and showed 0 make-up. That was ,my first concern new water coming into a system. There was this one time that a year old Vissman cast iron boiler packed up. They found that the janitor was doing a blowdown every day on the hotwater boiler!

hot_rod

good info, thanks. certainly water quality is part of that, regardless of flow velocities.

Henry

Yesterday's problem was caused when someone put the backup main circulating pump on. This caused twice the gpm going through the system and thus washing all the system pipes and radiators clean. This in turn blocked the bag filters who could not completly filter the water due to the velocity of the water and two caused two of three boilers to pack up and fail. The delta T for the system when I checked with the laser, was only 8F

Paul48

My thinking...If it's a backup it shouldn't be wired. There's ways to make it either, or, not both.