S PECIAL FEATURE

Specifying air-operated double-diaphragm pumps

Air- operated , double- d iaphragm pumps (AODDP) f i n d themselves in a host o f difficult pu mping applicw tions ranging, f r o m municipal sump p u m p i n g to high pur i t y operations. They have become the cost effective choice o f engineers, industry profe& sionals, and maintenance person. neL This is at tr ibuted to inherent

f e a t u r e s which inc lude: ease o f maintenance, variable speed, vat-# able pressure, ability o f sel~prime and run dry, d e a d h e a d capability to p a s s solids, a n d the absence o f leak p r o n e dynamic seals~ Due to the ease o f specifying an AODDP, it is easy to overlook a variety o f con. sideration.~ Long te~i~t satisfaction a n d high re turn on investment can be as sured i f reasonable care is exercised throughout the speciflca. tion proces~ This f ea ture aims to offer some guidelines, a n d is fog lowed by World Pumps" survey o f the principle manufacturers~

Materials o f construct ion

Two groups of p u m p componen ts which are pa r t i cu l a r l y i m p o r t a n t when specifying the pump ' s mater ia l of cons t ruct ion are the wetted par ts and the elastomers. Wetted par ts are

defined as the inlet & discharge manifolds, l iquid chambers, and outer d iaphragm pis ton plates.

Diaphragms, valve balls, valve seats and O-rings are collectively known as p u m p elastomers. One

m u s t cons ider four factors when specifying mater ia ls of construct ion: 1) Chemical compat-

ibility, 2) Abrasion, 3) Tempera ture l imitat ions, and 4) Ini t ial investment .

1) All wetted mater ia ls & elas tomers mus t

be chemica l ly compa t ib l e wi th the process

fluid, as well as c leaning solut ions used to

c lean the system. One can use a Chemical

Resistance Chart, available from the AODDP

manufac tu re r to assist in your selection efforts.

Gary J. Bowan, Wilden Pump & Engineering, Grand Terrace, CA, USA.

If the chemical is not listed in the Chemical

Chart, one can contact the chemical manufac-

tu re r for assistance. 2) The AODDP is able to handle viscous,

abras ive s lurr ies by des ign because of the

absence of metal to metal, contact. The p u m p uses two p u m p i n g chambers exhib i t ing low in te rna l velocities. However, abras ion can occur par t icular ly in the discharge ball cage area. One can prevent this from occurring by specifying a wetted mater ia l such as cast iron, 316 Stainless

Steel or Hastelloy with an e las tomer exhibi t ing abras ion resis tance such as polyurethane. Over- sizing the p u m p and r unn i ng it slow fur ther decreases the in te rna l velocity, resul t ing in long

componen t life. 3) Material t empera ture l imi ta t ions will vary

based on the specific material , consul t AODDP

m a n u f a c t u r e for l imi ta t ions . "Cold flow," a change in molecular structure, can occur in plastic pumps if operated outside s ta ted tem-

pera ture l imitat ions. The p u m p may become bri t t le or soft, creat ing a hazardous condit ion. Metallic pumps are usual ly l imited in tempera- ture by the p u m p elastomer specified. Elasto- mers may increase or decrease in size when

operated outside their t empera tu re range caus- ing unacceptab le working tolerances.

4) Ini t ial inves tment is always a considera- tion. Pump prices can vary by as much as 1000%

due to the choice of wetted cons t ruc t ion and elastomeric materials. One mus t consider all

aspects of the appl ica t ion and compare the Return On Inves tment to the ini t ial cost.

• Air distr ibution system

The air d is t r ibut ion system - the hear t of the p u m p - inc ludes the a i r valve a nd re la ted

components . This system reciprocates the p u m p by direct ing the air supply to one air chamber

and redirect ing systems available on the market.

WORLD PUMPS JANUARY 1997 Copyright ',~ 1997, Elsevier Science Ltd. All rights reserved. 0262 1762/97/$17.00

S PECIAL FEATURE

Some air valves are ex te rna l ly serviceable, with on ly one moving p a r t us ing only p r e s s u r e d i f ferent ia l to opera te . Others use bear ings , springs, magnets , and mechan ica l ass is tance. One mus t cons ider the ease of c leaning and r epa i r t ime needed af ter d i a p h r a g m failure. The var ie ty of a i r d i s t r ibu t ion systems is vast, and one mus t look at the t r ack record of the company and the success enjoyed by the users to evalua te i ts re l iabi l i ty and effectiveness.

Air consumpt ion

One mus t compare the a m o u n t of a i r requi red to pe r fo rm the app l i ca t ion vis-a-vis o the r AODDP manufac turers . This is accompl i shed by calcula t - ing the Total Dynamic Head (TDH) for the specific app l i ca t ion and p lo t t ing this f igure and the des i red gpm on the p u m p pe r fo rmance curve supp l i ed by the AODDP manufac turer . This TDH calcula t ion will t ake into account p re s su re losses a t t r i bu t ed to viscosity, specific gravity, and p ipe friction. Once the po in t is p lo t t ed on the curve, the a i r supp ly p re s su re (psi) and vo lume (scfm) can be derived. The volume of a i r r equ i red to ope ra t e the p u m p can be conver ted to horse- power by using the following calculat ions:

For p u m p supp ly p ressu re u n d e r 40 psi: X scfm x 0.125 = Horsepower

For pump supply pressures between 41-125 psi: X scfm x 0.25 = Horsepower

Choice o f check valves

Most AODDPs use 4 check valves. Two types widely ut i l ized are t he b a l l / s e a t conf igura t ion and the f lap valve type. The b a l l / s e a t configura- t ion exh ib i t s be t t e r pe r fo rmance charac te r i s t i c s c o m p a r e d to t he f lap valve type, i nc lud ing g rea te r vacuum capab i l i t i e s and longer compo- nen t life. The b a l l / s e a t des ign wears evenly because t he valve bal l sp ins when re leased and sea t s on the valve sea t in a d i f ferent pos i t ion on each stroke. The f lap valve, h inged in a f ixed posi t ion, f lexes con t inuous ly a t the same po in t which leads to fat igue and failure. Par t ic les in the p roces s f luid can eas i ly be t r a p p e d between the f lap valve and sea t causing d a m a g e to t he f lap wh ich l e a d s to p u m p per fo rmance concerns. I t is general ly sugges ted t h a t a b a l l / s e a t conf igura t ion be uti l ized. The f lap check valve shou ld only be usd when necessary to pass la rger solids. Ba l l / sea t config- u r a t i o n s can pa s s up to 1 3/8 inch sol ids (Wilden Model M20 - 4 inch inlet) .

[ ] Cost o f maintenance

For years AODDPs have been sub jec ted to ha r sh p u m p i n g du t i e s in ha r sh envi ronments . The AODDP excels in condi t ions where o the r types of p u m p s fail. Cont ra ry to p o p u l a r belief, t hese p u m p s do require main tenance . The e l a s tomers and a i r d i s t r i bu t ion sys tem requi re pe r iod ic inspec t ion and rep lacement . One should t ake into cons ide ra t ion the fact t h a t the p u m p will need to be m a i n t a i n e d and repai red . An inspec- t ion schedule should be der ived based on the specific app l i ca t ion pa ramete r s . Some manufac- tu re r s offer ease of ma in t enance th rough the use of c l amp band style fas teners which exped i t e th is process , while o the r s use a bo l ted flange. One should s tudy an e x p l o d e d view of the p u m p and acqua in t onesel f wi th the n u m b e r of pa r t s and r ep l acemen t p a r t cos ts of the AODDP in quest ion. Repair t ime and downt ime costs mus t also be considered.

AODDP manufac tu re r s offer accessor ies ex- pa nd ing the app l i ca t ion range of the AODDP. These a c c e s s o r i e s give t he a b i l i t y to t a k e advan tage of the i r inhe ren t charac te r i s t i c s in your mos t difficult appl ica t ions . Microprocessor based compu te r s are ab le to cont ro l the p u m p for b a t c h i n g a n d me te r ing . Leak d e t e c t i o n sy s t e ms a re a v a i l a b l e for c o m p l i a n c e wi th EPA and OSHA requi rements . High p re s su re p u m p s are avai lable for app l i ca t i ons requi r ing up to 300 psi d i scharge pressure . Surge dam- pe r s a re also avai lable which min imize pres- sure f luc tua t ion expe r i enced by these recipro- ca t ing pumps .

AODDPs are s imple by design, bu t r easonab le ca re in t h e s p e c i f i c a t i o n p r o c e s s m u s t be exerc ised to assure long t e rm sa t i s fac t ion and high Return On Investment . E duc a t e yourse l f via AODDP manufac tu re r s and the i r d i s t r ibu t ion ne twork to de t e rmine which AODDP is bes t for your appl ica t ion . Some AODDP manufac tu re r s offer de ta i l ed in format ion inc luding pe r fo rmance curves, e x p l o d e d views, pa r t s l is t ings, engineer- ing & ma in t enance manuals , videos, etc. Scruti- nize your local AODDP dis t r ibu tor . What are t he i r capabi l i t ies? Do they know the p roduc t? Can you expec t good service and fas t pa r t delivery?. Are they in the p u m p bus iness or do they sell the p u m p as a commodi ty i tem? Will they be a resource for you and your company?.

Specif icat ion is the key to p u m p performance. As with o the r engineered products , educa t ion and p r o p e r spec i f ica t ion will resu l t in long t e rm sat isfact ion. •

Contact: Wilden Pump & Engineering Com- pany, 22069 Van Buren Street, Grand Terrace, CA 92313, USA. Tel: +1 909 422 1730; Fax: + 1 909 783 3440.

WORLD PUMPS JANUARY 1997