<body><script type="text/javascript"> function setAttributeOnload(object, attribute, val) { if(window.addEventListener) { window.addEventListener('load', function(){ object[attribute] = val; }, false); } else { window.attachEvent('onload', function(){ object[attribute] = val; }); } } </script> <div id="navbar-iframe-container"></div> <script type="text/javascript" src="https://apis.google.com/js/plusone.js"></script> <script type="text/javascript"> gapi.load("gapi.iframes:gapi.iframes.style.bubble", function() { if (gapi.iframes && gapi.iframes.getContext) { gapi.iframes.getContext().openChild({ url: 'https://www.blogger.com/navbar.g?targetBlogID\x3d8968604820003269863\x26blogName\x3dChemical+%26+Process+Technology\x26publishMode\x3dPUBLISH_MODE_BLOGSPOT\x26navbarType\x3dBLUE\x26layoutType\x3dCLASSIC\x26searchRoot\x3dhttps://webwormcpt.blogspot.com/search\x26blogLocale\x3den\x26v\x3d2\x26homepageUrl\x3dhttp://webwormcpt.blogspot.com/\x26vt\x3d6505904876529004088', where: document.getElementById("navbar-iframe-container"), id: "navbar-iframe" }); } }); </script>

Chemical Process Technology

Continue to learn tips, knowledge and experience about Chemical Process Technology...

Enter your email address:



Chemical & Process Technology

A place to share knowledge, lesson learnt...

Sunday, February 17, 2008

Display problem ? Click HERE

Why centrifugal pump Net Positive Suction Head required (NPSHr) increases with flow ?

There is a simple answer. NPRHr is a parameter fixed during design and fabrication of a centrifugal pump. The NPSHr is increased with flow. Main reason to this increase is increase in frictional lose at entrances nozzle and turbulence friction lose at vane tip.

Recommended UTILITY :
Clipmarks - Useful Utility for Blogger and Publisher

Let's look at the following image.

The pressure at different point location from entrance pipe to impeller is indicated as red line in the curve for normal flow. From point A-C, the friction lose cause by nozzle whilst turbulence friction lose from point C-D at impeller center. The pressure increases from D-E as impeller is adding head into the liquid. As the flow increases from normal flow and the physical configuration is fixed, the frictional lose and turbulence friction lose will increase with flow. The pressure profile from entrance nozzle to pump impeller center and outlet will be shifted downward as indicated in blue line. The lowest pressure at the impeller center is getting closer or even cross the vapor pressure of the fluid. This potentially results cavitation.

Pump manufacturer would needs to design a pump with the Suction specific speed (Nss) below 10000 (normally) to **ensure a healthy life span as high Nss would leads to rough operation and shorten life span. Some manufacturer may even design for 8000-8500 .

Suction specific speed ( Nss) of a pump is a dimensionless number expressed as

Nss=( N* Q 0.5 ) / (NPSHr)0.75

Nss : Suction Specific speed
Q : Flow rate (gpm) at the Best Efficiency Point
N : Pump rotational speed (rpm)
NPSHr : Net Positive Suction Head required (ft)
Since the normal pump speed is 1450 rpm for low speed pump and 2950 rpm for high speed pump, it is obvious that the NPSHr is increased as flow rate increased.

Updated on Feb 21, 2008 : ***

Related Topic

Continue receive FREE stuff & Update.......Subscribe by Email OR RSS
Any comments / advices ?..........Drop it in comment field (below) or send a private email to me...


posted by Webworm, 1:51 PM


Post a Comment

Let us know your opinion !!! You can use some HTML tags, such as  <b>, <i>, <a>

Subscribe to Post Comments [Atom]

Links to this post:

Create a Link


<< Home