R.L. Deppmann Company

Pump Suction Piping Rules – Cooling Tower Pumps and Piping – Part 6

Norm Hall — Mon, 14 May 2012 04:06:41 +0000

Over the last several weeks we discussed the feared NPSH and showed it is rarely an issue at design conditions. More often than not, noise and pressure problems are caused by air in the suction piping system. The suction pipe, in many cooling pumping systems, is under very little pressure. In fact, the suction of the pump could be under a vacuum.

AIR WILL NOT VENT FROM A VACUUM TO ATMOSPHERE!

Bell and Gossett has several piping rule suggestions for cooling tower pump suctions.

RECOMMENDATION #1: Always specify an anti-vortex baffle, sometimes called a doghouse, in the tower outlets.

Towers normally have very low storage heights of water in the basin. Any vortex that starts will reduce the flow rate and pull air into the piping system.

RECOMMENDATION #2: Avoid air traps caused by elevation changes in the suction pipe. Elbows used to go up & over pipe or obstacles cause an air trap. The improper use of a reducer at the pump flange can have the same effect.

RECOMMENDATION #3: When needed, use eccentric reducers at the pump suction. The orientation depends on where the suction pipe is located.

In both examples to the right, we avoid trapping air while transitioning pipe sizes.

Next week we will continue to look at cooling tower pump suctions.

Click here to request a copy of the Xylem Bell and Gossett
Cooling Tower Piping technical bulletin TEH-1209

Disclaimer: R. L. Deppmann and it’s affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions that result from this information. Please feel free to contact us if you do have any questions.

Archives – Click here for Past Articles

It’s all about Pressure – Cooling Tower Pumps and Piping – Part 5

Norm Hall — Mon, 07 May 2012 11:01:03 +0000

Printer Friendly (PDF)

Last week the R. L. Deppmann Monday Morning Minute gave an example of a pump selection for which we applied the margin multipliers to arrive at 24.5 feet of net positive suction head required or NPSH_R. Now let’s turn our attention to the cooling tower pump suction piping and net positive suction head available or NPSH_A.

Look at the cooling tower system representation in the figure below. I’ll give the formula and then I’ll give you an easy chart that Xylem Bell and Gossett created. The NPSH_A at the pump suction can be calculated using the formula.

NPSH_A = Absolute atmospheric pressure in feet (34’ at sea level) + elevation in feet – pipe friction loss in feet – strainer and valve pressure drop in feet – vapor pressure of the liquid in feet.

OK, now let’s make it easier for you! The Bell and Gossett TEH-1209 training publication on Cooling Tower Pumping and Piping has a great graph you can use. It is shown below in figure 2.

Example 1: Let’s assume the elevation is 8 feet, the pipe is 10” and is 50 feet long (in a later article we will tell you not to use 10”), the strainer has a pressure drop of 3.5 feet clean, and the butterfly valve pressure drop is negligible. The calculated gauge pressure will become (0# + (8/2.31) – (2/2.31) – (3.5/2.31) = 1.08 PSIG.
Now looking at the B&G chart figure 18. We enter at 1 PSIG on the bottom and go up to 85°F water, then across to the vertical specific gravity line of 1 and read about 32 feet of NPSH available. Since 32 is greater than the 24.5 needed, there is no problem with NPSH.

Example 2: Let’s change the elevation to 2 feet and keep everything else the same. Our formula for gauge pressure will be: (0# + (2/2.31) – (2/2.31) – (3.5/2.31) = -1.5 PSIG or about 4 inches of vacuum. Let’s look at the chart again. Going up from (-1.5) to 85°F water then across till we intersect the vertical specific gravity line of 1, we read about 30 feet of NPSH available. The pump suction is in a vacuum, yet we are still OK on NPSH.

NPSH at design flow rate is rarely an issue in tower systems. The noise and operational problems come in because of piping, air, excessive pressure drop devices, and drift on the curve. We will look at these in the next several weeks.

Next week we look at cooling tower pump suctions.

Click here to request a copy of the Xylem Bell and Gossett
Cooling Tower Piping technical bulletin TEH-1209

Archives – Click here for Past Articles

It’s all about Pressure – Cooling Tower Pumps and Piping – Part 4

Norm Hall — Mon, 30 Apr 2012 04:05:00 +0000

Printer Friendly (PDF)

NOTE: An error was found in our last Monday Morning Minute and corrected at 9:30 AM on 4-23-12. If you printed the MMM before that time, please go back and re-print.

Last week the R. L. Deppmann Monday Morning Minute left you with a multiplier of 1.3, 1.5, or 2.0 to use with the pump NPSH_R. Let’s look at an example this week using the suction energy formula we introduced last week and add one more margin to the formula.

Let’s start with the additional margin. Tower water has a great deal of air entrained and that air can play havoc with the gauge pressure at the pump suction. How often have you witnessed the bouncing gauge in this situation? This is the reason I add 5 extra feet to the NPSHR as a safety factor. You may choose to add this or not, but I always start by adding it in. Now let’s look at an example. I suggest you have the last Monday Morning minute handy.

Example: Cooling Tower Pump for 3000 GPM at 80 feet using a B&G series VSX-VSCS-8x10x10.5A

D = suction size = 10″

n = speed = 1780

Q = 3514 GPM at BEP and max. Impeller/2 = 1757 because this is a double suction impeller.

NPSH = 20 at BEP and max impeller.

So from the last MMM: Suction Energy = D x n x (RPM x Q.5 /NPSHR.75) X10-6

So Suction Energy = 10 x 1780 x (1780 x (1757) .5 / (20) .75) x (.000001)≈ 140 which is a low energy pump.

Now, from the chart published in the last Monday Morning Minute, the multiplier to use with published NPSH_R will be 1.3 and the recommended safety factor from statements above will be 5 feet. The NPSH_R at 3000 GPM is 15 feet absolute from the curve above. We will want our available NPSH to be [[15 x 1.3 = 19.5 + 5 = 24.5]] feet absolute or greater.

Next week we turn out attention to the suction piping of the cooling tower pump and available NPSH…

Click here to request a copy of the Xylem Bell and Gossett
Cooling Tower Piping technical bulletin TEH-1209

Archives – Click here for Past Articles

Where cavitation begins – Cooling Tower Pumps and Piping – Part 3

Norm Hall — Mon, 23 Apr 2012 04:22:04 +0000

Printer Friendly (PDF)

Last week the R. L. Deppmann Monday Morning Minute defined NPSH_R, and ended with the Hydraulic Institute (HI) definition as the absolute pressure that will cause the total head of the pump to be reduced by 3%, due to flow blockage from cavitation”. Of importance is the fact that it does not say that NPSH_R is where cavitation begins.

The place where cavitation begins is called incipient cavitation and can be from 2 to 20% greater than the NPSH_R reported on the pump curve. In addition, NPSH_R is a tested value, the test is with clear un-aerated water; not dirty, highly aerated, tower water and pumps have manufacturing tolerances. The NPSHR can also vary with the system fluid conditions.

So to reduce the chance of damage to the pump from cavitation, we need to use a larger number than the pump net positive suction head required. A few years ago HI defined some margins to apply to the published pump NPSH_R with different systems and pump types. The margin depended on the application and the SUCTION ENERGY of the pumps. For cooling tower applications, the recommendation was to use a multiplier to NPSH_R of 1.3 for low energy pumps, 1.5 for high energy pumps, and 2.0 for very high energy pumps.

SO HOW DO WE DETERMINE WHICH MARGIN TO USE?

The suction energy of a pump depends on a number of variables but it can be approximated by the formula:

Suction Energy= D x n x S X 10 ^(-6)
Where S = Suction Specific Speed or (RPM X Q^(.5) / NPSH_R^(.75) )

D = Pump suction size (in)

n = Speed

Q = Flow at Best Efficiency Point (BEP) for the maximum impeller

NPSH is at the flow at BEP for maximum impeller also.

Note: that for double suction pumps, you use ½ of the flow at BEP

Now you use the result to choose the margin using this chart.

Suction Energy	Pump Type	Margin for Cooling Towers
Less than 150-220	Low Energy	1.3
Between 150-220 and 250-380	High Energy	1.5
Over 250-380	Very High Energy	2.0

Next week we use an example and complete this discussion of Margin applied to NPSHR.

Click here to request a copy of the Xylem Bell and Gossett
Cooling Tower Piping technical bulletin TEH-1075

Archives – Click here for Past Articles

It’s all about Pressure – Cooling Tower Pumps and Piping – Part 2

Star Scott — Mon, 16 Apr 2012 04:55:23 +0000

Printer Friendly (PDF)

Here at R. L. Deppmann, we receive a continuous barrage of questions regarding concern over NPSH (net positive suction head) and tower pump selections. It is the most confusing set of values shown on a pump curve. This week, let’s look at NPSH_R or net positive suction pressure required by the pump.

Net positive suction head is the amount of ABSOLUTE pressure required inside the pump suction to make sure the water remains water and does not flash into vapor. Water boils into vapor at 212°F at 0 PSIG. We do this every time we put a pot on the stove to make pasta. Most tower systems are designed to create a maximum of 85°F. Water will not boil at 85°F unless we put it under a substantial gauge vacuum. So NPSH_R is expressed in feet of ABSOLUTE pressure, not gauge pressure. Let’s look at an example.

Here is an example of a pump selection using a Bell and Gossett side suction, top discharge pump designed for 3000 GPM at 70 feet of head.

The NPSH_R curve is in red at the bottom. If you read up from 3000 GPM to that curve and across to the right, the NPSH_R is 15 feet of absolute pressure or about 16” on a vacuum gauge.

Here is a curve you need to understand when looking at NPSH_R. The Hydraulic Institute (HI), in their publication of standards, defines as the a NPSH that will cause the total head of the pump to be reduced by 3%, due to flow blockage from cavitation. It does not say that NPSH_R is where cavitation begins. So what do we do with a statement like that!

Next week we continue this discussion of NPSH_R.

Click here to request a copy of the Xylem Bell and Gossett
Cooling Tower Piping technical bulletin TEH-1075

Archives – Click here for Past Articles

It’s all about starting with the terms! – Cooling Tower Pumps and Piping – Part 1

Norm Hall — Mon, 09 Apr 2012 04:40:17 +0000

Printer Friendly (PDF)

Most cooling tower designs and installations are commissioned without any issues. The few percent of times where problems arise will cause multiple meetings, emails, finger pointing, and added costs. The issues are not caused by bad luck or bad Karma; tower water pumping issues are typically caused by air, dirt, or improper application.

Over the next few weeks, the R. L. Deppmann Monday Morning Minutes blog will focus on the design and installation of cooling tower piping and pumps. Let’s start with a few terms.

PSIG: The gauge pressure in pounds per square inch.

PSIA: The absolute pressure in pounds per square inch. When a gauge reads “0” at sea level, there is 14.7 PSIA or 34 feet absolute available. This is used in NPSH calculations.

NPSH_A: Net positive suction pressure available is the absolute pressure expressed in feet at the pump suction.

NPSH_R: Net positive suction pressure required is the absolute pressure expressed in feet required in the pump to keep the fluid from flashing.

Static Pressure: The pressure expressed in PSI or feet caused by the column of water above the reading.

Open System: A hydronic system where the reference pressure is “0” PSIG (at sea level) caused by a part of the system open to atmosphere.

Next week we begin by looking at the suction piping at the tower.

Click here to request a copy of the Xylem Bell and Gossett
Cooling Tower Piping technical bulletin TEH-1075

Archives – Click here for Past Articles

Absorption Distance In Action

Chris Lieder — Mon, 26 Mar 2012 13:26:57 +0000

Printer Friendly (PDF)

In the last couple Monday Morning Minutes, we showed how the type of humidifier dispersion method could affect the absorption distance. Let’s look at an example.

Ex: 5000 CFM at 55OF Duct temperature to provide a room RH at 35% at 70OF with 50% outside air.

Traditional single tube dispersion……… 59” inches absorption
Rapidsorb multiple tube dispersion..….. 22” inches absorption
Ultrasorb panel type dispersion………… 6” inches absorption

As you can see, in tight spaces, Ultrasorb or Rapidsorb can solve wetting problems in ducts.

Next week we show an easy way to save energy.

– View Previous Post –

Archives – Click here for Past Articles

Humidifier Dispersion Choices And Absorption Distance

Chris Lieder — Mon, 19 Mar 2012 13:57:03 +0000

Printer Friendly (PDF)

Last week, we described the criteria that may affect absorption distance. This week we look at how your choice of humidifier dispersion assembles may affect that absorption distance. Call your R.L. Deppmann sales representative to assist you in selection of the best dispersion method for your application.

space
Figure 58-1 the DRI-STEEM Humidification System Design Guide.

Next week we show an example of absorption distance.

– View Previous Post –

Archives – Click here for Past Articles

Humidifier Placement In Airstream is Critical

Chris Lieder — Mon, 12 Mar 2012 13:10:57 +0000

Printer Friendly (PDF)

Absorption or non-wetting distance is the dimension downstream from the leaving side of the steam dispersion assembly to the point where wetting will not occur, although wisps of steam may be present. Solid objects at duct air temperature, such as coils, dampers, fans, etc., downstream of this dimension will remain dry.

Location A is the best choice.
Installing downstream form heating and cooling coils provides laminar flow through the dispersion unit; plus, the heated air provides an environment of best absorption.
Location B is the second-best choice.
However, in change-over periods, the cooling coil will eliminate some moisture for humidification.
Location C is the third-best choice.
Air leaving a fan is usually very turbulent and can cause vapor to not absorb at the expected non-wetting distance. Allow for more distance if installing downstream form a fan.
Location D is the poorest choice.
The cooler air at this location requires an increased non-wetting distance.

Determine humidifier placement
A humidification system generally consists of a vapor or steam generator and a dispersion assembly. The proper placement of these two components is crucial for successful system operation. Usually there is no single correct placement for a humidifier. Much depends on system design and application. However, the following paragraphs and dispersion assembly placement examples are presented as guidelines for common situations.

First check available absorption distance
Available absorption distance affects system choice. Dispersed steam must be absorbed into the airflow before it comes in contact with any duct elbows, fans, vanes, filters, or any object that can cause condensation and dripping. Not all humidification systems guarantee absorption within a short distance, so it is important to be aware of the available absorption distance. It is also important to know coil dimensions vs. inside air handling unit dimensions for proper sizing of dispersion assemblies.

Next week we look at humidifier dispersion assemblies.

– View Previous Post –

Archives – Click here for Past Articles

Factors That Affect Absorption – Humidification Part II

Chris Lieder — Mon, 05 Mar 2012 04:59:49 +0000

Printer Friendly (PDF)

space

Duct or AHU temperature. Cool air absorbs less than warm air and requires a longer absorption distance.

When equal amounts of steam are introduced into equivalent ducts but with different air temperatures, the lower temperature systems of 50 °F to 55 °F are more difficult to ensure absorption than systems with higher temperatures.
Δ RH (the difference between entering and leaving RH). The more vapor that needs to be dispersed into the airstream, the longer the absorption distance.

In general, the higher the relative humidity or load that must be dispersed in the airstream the more challenging it is to control absorption distance.
Mixing of air and steam. Uneven airflow, non-uniform mixing of steam with air, and the number of steam discharge points on a dispersion assembly affect absorption distance. In general the more tubes with the airstream the shorter the absorption distance.

Next week we look at placement in the airstream.

– View Previous Post –

Archives – Click here for Past Articles